A word at the start concerning the choice and purchase of telescopes. The question of refractors vs. reflectors has been already considered. The outcome of the case depends on how much and how often you are likely to use the instrument, and just what you want it for. For casual observations and occasional use—all that most busy buyers of telescopes can expect—the refractor has a decided advantage in convenience. If one has leisure for frequent observations, and particularly if he can give his telescope a permanent mount, and is going in for serious work, he will do well not to dismiss the idea of a reflector without due deliberation.

In any case it is good policy to procure an instrument from one of the best makers. And if you do not buy directly of the actual maker it is best to deal with his accredited agents. In other words avoid telescopes casually picked up in the optical trade unless you chance to have facilities for thorough testing under competent guidance before purchase. No better telescopes are made than can be had from the best American makers. A few British and German makers are quite in the same class. So few high grade French telescopes reach this country as to cause a rather common, but actually unjust,[28] belief that there are none.

If economy must be enforced it is much wiser to try to pick up a used instrument of first class manufacture than to chance a new one at a low price. Now and then a maker of very ordinary repute may turn out a good instrument, but the fact is one to be proved—not assumed. Age and use do not seriously deteriorate a telescope if it has been given proper care. Some of Fraunhofer’s are still doing good service after a century, and occasionally an instrument from one of the great makers comes into the market at a real bargain. It may drift back to the maker for resale, or turn up at any optician’s shop, and in any case is better worth looking at than an equally cheap new telescope.

The condition of the tube and stand cuts little figure if they are mechanically in good shape. Most of the older high grade instruments were of brass, beautifully finished and lacquered, and nothing looks worse after hard usage. It is essential that the fitting of the parts should be accurate and that the focussing rack should work with the utmost smoothness. A fault just here, however, can be remedied at small cost. The mount, whatever its character, should be likewise smooth working and without a trace of shakiness, unless one figures on throwing it away.

As to the objective, it demands very careful examination before a real test of its optical qualities. The objective with its cell should be taken out and closely scrutinized in a strong light after the superficial dust has been removed with a camel’s hair brush or by wiping very gently with the soft Japanese “lens paper” used by opticians.

One is likely to find plenty to look at; spots, finger marks, obvious scratches, and what is worse a network of superficial scratches, or a surface with patches looking like very fine pitting. These last two defects imply the need of repolishing the affected surface, which means also more or less refiguring. Ordinary brownish spots and finger marks can usually be removed with little trouble.

The layman, so to speak, is often warned never to remove the cell from a telescope but he might as well learn the simpler adjustments first as last. In taking off a cell the main thing is to see what one is about and to proceed in an orderly manner. If the whole cell unscrews, as often is the case in small instruments, the only precaution required is to put a pencil mark on the cell and its seat so that it can be screwed back to where it started.

If as is more usual the cell fits on with three pairs of screws, one of each pair will form an abutment against which its mate pulls the cell. A pencil mark locating the position of the head of each of the pulling screws enables one to back them out and replace them without shifting the cell.

The first inspection will generally tell whether the objective is worth further trouble or not. If all surfaces save the front are in good condition it may pay to send the objective to the maker for repolishing. If more than one surface is in bad shape reworking hardly pays unless the lens can be had for a nominal figure. In buying a used instrument from its original source these precautions are needless as the maker can be trusted to stand back of his own and to put it in first class condition.

However, granted that the objective stands well the inspection for superficial defects, it should then be given a real test for figure and color correction, bearing in mind that objectives, even from first class makers, may now and then show slightly faulty corrections, while those from comparatively unknown sources may now and then turn out well. In this matter of necessary testing old and new glasses are quite on all fours save that one may safely trust the maker with a well earned reputation to make right any imperfections. Cleansing other than dusting off and cautiously wiping with damp and then dry lens paper requires removal of the lenses from their cell which demands real care.

With a promising looking objective, old or new, the first test to be applied is the artificial star—artificial rather than natural since the former stays put and can be used by day or by night. For day use the “star” is merely the bright reflection of the sun from a sharply curved surface—the shoulder of a small round bottle, a spherical flask silvered on the inside, a small silvered ball such as is used for Christmas tree decoration, a bicycle ball, or a glass “alley” dear to the heart of the small boy.

The object, whatever it is, should be set up in the sun against a dark background distant say 40 or 50 times the focal length of the objective to be tested. The writer rather likes a silvered ball cemented to a big sheet of black cardboard. At night a pin hole say 1/32 inch or less in diameter through cardboard or better, tinfoil, with a flame, or better a gas filled incandescent lamp behind it, answers well. The latter requires rather careful adjustment that the projected area of the closely coiled little filament may properly fill the pinhole just in front of it.

Now if one sets up the telescope and focusses it approximately with a low power the star can be accurately centered in the field. Then if the eyepiece is removed, the tube racked in a bit, and the eye brought into the focus of the objective, one can inspect the objective for striÆ. If these are absent the field will be uniformly bright all over. Not infrequently however one will see a field like Fig. 152 or Fig. 153. The former is the appearance of a 4 inch objective that the author recently got his eye upon. The latter shows typical striÆ of the ordinary sort. An objective of glass as bad as shown in Fig. 152 gives no hope of astronomical usefulness, and should be relegated to the porch of a seashore cottage. Figure 153 may represent a condition practically harmless though undesirable.

The next step is a really critical examination of the focal image. Using a moderately high power ocular, magnifying say 50 to the inch of aperture, the star should be brought to the sharpest focus possible and the image closely examined. If the objective is good and in adjustment this image should be a very small spot of light, perfectly round, softening very slightly in its brilliancy toward the edge, and surrounded by two or three thin, sharp, rings of light, exactly circular and with well defined dark spaces separating them.

Often from the trembling of the air the rings will seem shaky and broken, but still well centered on the star-disc. The general appearance is that shown in Fig. 154.[29]

Instead, several very different appearances may turn up. First, the bright diffraction rings may be visible only on one side of the central disc, which may itself be drawn out in the same direction. Second, the best image obtainable may be fairly sharp but angular or irregular instead of round or oval and perhaps with a hazy flare on one side. Third, it may be impossible to get a really sharp focus anywhere, the image being a mere blob of light with nothing definite about it.

One should be very sure that the eyepiece is clean and without fault before proceeding further. As to the first point a bit of lens paper made into a tiny swab on a sliver of soft wood will be of service, and the surfaces should be inspected with a pocket lens in a good light to make sure that the cleaning has been thorough. Turning the ocular round will show whether any apparent defects of the image turn with it.

In the first case mentioned the next step is to rack the ocular gently out when the star image will expand into a more or less concentric series of bright interference rings separated by dark spaces, half a dozen or so resulting from a rather small movement out of focus. If these rings are out of round and eccentric like Fig. 155 one has a clear case of failure of the objective to be square with the tube, so that the ocular looks at the image askew.

In the ordinary forms of objective this means that the side of the objective toward the brighter and less expanded part of the ring system is too near the ocular. This can be remedied by pushing that side of the objective outwards a trifle. Easing off the pulling screw on that side and slightly tightening the abutment screw makes the needed correction, which can be lessened if over done at the first trial, until the ring system is accurately centered. It is a rather fussy job but not at all difficult if one remembers to proceed cautiously and to use the screw driver gently.

In the second case, racking out the ocular a little gives a ring system which exaggerates just the defects of the image. The faults may be due to mechanical strain of the objective in its cell, which is easily cured, or to strains or flaws in the glass itself, which are irremediable. Therefore one should, with the plane of the objective horizontal, loosen the retaining ring that holds the lenses, without disturbing them, and then set it back in gentle contact and try the out of focus rings once more. If there is no marked improvement the fault lies in the glass and no more time should be wasted on that particular objective. Fig. 156 is a typical example of this fault.

In dealing with case three it is well to give the lens a chance by relieving it of any such mechanical strains, for now and then they will apparently utterly ruin the definition, but the prognosis is very bad unless the objective has been most brutally mishandled.

In any case failure to give a sharply defined focus in a very definite plane is a warning that the lens (or mirror) is rather bad. In testing a reflector some pains must be taken at the start with both the main and the secondary mirror. Using an artificial star as before, one should focus and look sharply to the symmetry of the image, taking care to leave the instrument in observing position and screened from the sun for an hour or two before testing. Reflectors are much more sensitive to temperature than refractors and take longer to settle down to stability of figure. With a well mounted telescope of either sort a star at fair altitude on a fine night gives even better testing conditions than an artificial star, (Polaris is good in northern latitudes) but one may have a long wait.

If the reflector is of good figure and well adjusted, the star image, in focus or out, has quite the same appearance as in a refractor except that with a bright star in focus one sees a thin sharp cross of light centered on the image, rather faint but perfectly distinct. This is due to the diffraction effect of the four thin strips that support the small mirror, and fades as the star is put out of focus.

The rings then appear as usual, but also a black disc due to the shadowing of the small mirror. Fig. 157 shows the extra-focal image of a real or artificial star when the mirror is well centered, and the star in the middle of the field. There only are the rings round and concentric with the mirror spot. The rings go out of round and the spot out of center for very small departure from the middle of the field when the mirror is of large relative aperture—F/5 or F/6.

If the star image shows flare or oval out-of-focus rings when central of the field, one or both mirrors probably need adjustment. Before laying the trouble to imperfect figure, the mirrors should be adjusted, the small one first as the most likely source of trouble. The side of the mirror toward which the flare or the expanded side of the ring system projects should be slightly pushed away from the ocular. (Note that owing to the reflection this movement is the reverse of that required with a refractor.)

If the lack of symmetry persists one may as well get down to first principles and center the mirrors at once. Perhaps the easiest plan is to prepare a disc of white cardboard exactly the size of the mirror with concentric circles laid out upon it and an eighth inch hole in the center. Taking out the ocular and putting a half inch stop in its place one can stand back, lining up the stop with the draw tube, and see whether the small mirror looks perfectly round and is concentric with the reflected circles. If not, a touch of the adjusting screws will be needed.

Then with a fine pointed brush dot the center of the mirror itself through the hole, with white paint. Then, removing the card, one will see this dot accurately centered in the small mirror if the large one is in adjustment, and it remains as a permanent reference point. If the dot be eccentric it can be treated as before, but by the adjusting screws of the large mirror.

The final adjustment can then be made by getting a slightly extra-focal star image fairly in the center of the field with a rather high power and making the system concentric as before described. This sounds a bit complicated but it really is not. If the large mirror is not in place, its counter cell may well be centered and levelled by help of a plumb line from the center of the small mirror and a steel square, as a starting point, the small mirror having been centered as nearly as may be by measurement.[30]

So much for the general adjustment of the objective or mirror. Its actual quality is shown only on careful examination.

As a starting point one may take the extra-focal system of rings given by an objective or mirror after proper centering. If the spherical aberration has thoroughly removed the appearance of the rings when expanded so that six or eight are visible should be like Fig. 158. The center should be a sharply defined bright point and surrounding it, and exactly concentric, should be the interference rings, truly circular and gradually increasing in intensity outwards, the last being very noticeably the strongest.

One can best make the test when looking through a yellow glass screen which removes the somewhat confusing flare due to imperfect achromatism and makes the appearances inside and outside focus closely similar. Just inside or outside of focus the appearance should be that of Fig. 159 for a perfectly corrected objective or mirror.

Sometimes an objective will be found in which one edge of the focussed star image is notably red and the opposite one tinted with greenish or bluish, showing unsymmetrical coloring, still more obvious when the image is put a little out of focus. This means that the optical centers of crown and flint are out of line from careless edging of the lenses or very bad fitting. The case is bad enough to justify trying the only remedy available outside the optician’s workshop—rotating one lens upon the other and thus trying the pair in different relative azimuths.

The initial positions of the pair must be marked plainly, care must be taken not to displace the spacers 120° apart often found at the edges of the lenses, and the various positions must be tried in an orderly manner. One not infrequently finds a position in which the fault is negligible or disappears altogether, which point should be at once marked for reference.

In case there is uncorrected spherical aberration there is departure from regular gradation of brightness in the rings. If there is a “short edge,” i.e., + spherical aberration, so that rays from the outer zone come to a focus too short, the edge ring will look too strong within focus, and the inner rings relatively weak; with this appearance reversed outside focus. A “long edge” i.e., - spherical aberration, shows the opposite condition, edge rings too strong outside focus and too weak within. Both are rather common faults. The “long edge” effect is shown in Figs. 160 and 161, as taken quite close to focus.

It takes a rather sharp eye and considerable experience to detect small amounts of spherical aberration; perhaps the best way of judging is in quickly passing from just inside to just outside focus and back again, using a yellow screen and watching very closely for variations in brightness. Truth to tell a small amount of residual aberration, like that of Fig. 160, is not a serious matter as regards actual performance—it hurts the telescopist’s feelings much more than the quality of his images.

A much graver fault is zonal aberration, where some intermediate zone of objective or mirror comes to a focus too long or too short, generally damaging the definition rather seriously, depending on the amount of variation in focus of the faulty zone. A typical case is shown in Fig. 162 taken within focus. Here two zones are abnormally strong showing, just as in the case of simple spherical aberration, too short focus. Outside of focus the intensities would change places, the outer and midway zones and center being heavy, and the strong zones of Fig. 162 weak. These zonal aberrations are easily detected and are rather common both in objectives and mirrors, though rarely as conspicuous as in Fig. 162.

Another failing is the appearance of astigmatism, which, broadly, is due to a refracting or reflecting surface which is not a surface of revolution and therefore behaves differently for rays incident in different planes around its optical axis. In its commonest form the surface reflects or refracts more strongly along one plane than along another at right angles to it. Hence the two have different foci and there is no point focus at all, but two line foci at right angles. Figs. 163 and 164 illustrate this fault, the former being taken inside and the latter outside focus, under fairly high power. If a star image is oval and the major axis of this oval has turned through 90° when one passes to the other side of focus, astigmatism is somewhere present.

As more than half of humanity is astigmatic, through fault of the eye, one should twist the axis of the eyes some 90° around the axis of the telescope and look again. If the axis of the oval has turned with the eyes a visit to the oculist is in order. If not, it is worth while rotating the ocular. If the oval does not turn with it that particular telescope requires reworking before it can be of much use.

This astigmatism due to fault of figure must not be confused with the astigmatic difference of the image surfaces referred to in Chapter IV which is zero on the axis and not of material importance in ordinary telescopes. Astigmatism of figure on the contrary is bad everywhere and always. It should be especially looked out for in reflecting surfaces, curved or plane, since it is a common result of flexure.

Passing on now from these simple tests for figure, chromatic aberration has to be examined. Nothing is better than an artificial star formed by the sun in daylight, for the preliminary investigation. At night Polaris is advantageous for this as for other tests.

The achromatization curves, Fig. 163, really tell the whole story of what is to be seen. When the telescope is carefully focussed for the bright part of the spectrum, getting the sharpest star image attainable, the central disc, small and clean, should be yellowish white, seen under a power of 60 or 70 per inch of aperture.

But the red and blue rays have a longer focus and hence rim the image with a narrow purplish circle varying slightly in hue according to the character of the achromatization. Pushing the ocular a little inside, focus, the red somewhat overbalances the blue and the purple shades toward the red. Pulling out the ocular very slightly one brings the deep red into focus as a minute central red point, just as the image begins to expand a little. Further outside focus a bluish or purplish flare fills the center of the field, while around it lies a greenish circle due to the rays from the middle of the secondary spectrum expanding from their shorter focus.

In an under-corrected objective this red point is brighter and the fringe about the image, focussed or within focus, is conspicuously reddish. Heavy overcorrection gives a strong bluish fringe and the red point is dull or absent. With a low power ocular, unless it be given a color correction of its own, any properly corrected objective will seem under-corrected as already explained.

The color correction can also be well examined by using an ocular spectroscope like Fig. 140, with the cylindrical lens removed. Examining the focussed star image thus, the spectrum is a narrow line for the middle color of the secondary spectrum, widening equally at F and B, and expanding into a sort of brush at the violet end. Conversely, when moved outside focus until the width is reduced to a narrow line at F and B, the widening toward the yellow and green shows very clearly the nature and extent of the secondary spectrum. In this way too, the actual foci for the several colors can easily be measured.

The exact nature of the color correction is somewhat a matter of taste and of the uses for which the telescope is designed, but most observers agree in the desirability of the B-F correction commonly used as best balancing the errors of eye and ocular. With reflectors, achromatic or even over-corrected oculars are desirable. The phenomena in testing a telescope for color vary with the class of star observed—the solar type is a good average. Trying a telescope on [alpha] LyrÆ emphasizes unduly the blue phases, while [alpha] Orionis would overdo the red.

The simple tests on star discs in and out of focus here described are ample for all ordinary purposes, and a glass which passes them well is beyond question an admirably figured one. The tests are not however quantitative, and it takes an experienced eye to pick out quickly minor errors, which are somewhat irregular. One sometimes finds the ring system excellent but a sort of haze in the field, making the contrasts poor—bad polish or dirt, but figure good.

A test found very useful by constructors or those with laboratory facilities is the knife edge test, worked out chiefly by Foucault and widely used in examining specula. It consists in principle of setting up the mirror so as to bring the rays to the sharpest possible focus. For instance in a spherical mirror a lamp shining through a pin hole is placed in the centre of curvature, and the reflected image is brought just alongside it where it can be inspected by eye or eyepiece. In Fig. 165 all the rays which emanate from the pinhole b and fall on the mirror a are brought quite exactly to focus at c. The eye placed close to c will see, if the mirror surface is perfect, a uniform disc of light from the mirror.

If now a knife edge like d, say a safety razor blade, be very gradually pushed through the focus the light will be cut off in a perfectly uniform manner—no zone or local spot going first. If some error in the surface at any point causes the reflected ray to miss the focus and cross ahead of or behind it as in the ray bef, then the knife edge will catch it first or last as the case may be, and the spot e will be first darkened or remain bright after the light elsewhere is extinguished.

One may thus explore the surface piecemeal and detect not only zones but slight variations in the same zone with great precision. In case of a parabolic mirror as in Fig. 166 the test is made at the focus by aid of the auxiliary plane mirror, and a diagonal as shown, the pinhole and knife edge being arranged quite as before. A very good description of the practical use of the knife edge test may be found in the papers of Dr. Draper and Mr. Ritchey already cited.

It is also applied to refractors, in which case monochromatic light had better be used, and enables the experimenter to detect even the almost infinitesimal markings sometimes left by the polishing tool, to say nothing of slight variations in local figure which are continually lost in the general illumination about the field when one uses the star test in the ordinary manner.

The set-up for the knife edge experiments should be very steady and smooth working to secure precise results, and it therefore is not generally used save in the technique of figuring mirrors, where it is invaluable. With micrometer motions on the knife edge, crosswise and longitudinally, one can make a very exact diagnosis of errors of figure or flexure.

A still more delicate method of examining the perfection of figuring is found in the Hartmann test. (Zeit. fur Instk., 1904, 1909). This is essentially a photographic test, comparing the effect of the individual zones of the objective inside and outside of focus. Not only are the effects of the zones compared but also the effects of different parts of the same zone, so that any lack of symmetry in performance can be at once found and measured.

The Hartmann test is shown diagrammatically in Fig. 167. The objective is set up for observing a natural or artificial star. Just in front of it is placed an opaque screen perforated with holes, as shown in section by Fig. 167, where A is the perforated screen. The diameters of the holes are about 1/20 the diameter of the objective as the test is generally applied, and there are usually four holes 90° apart for each zone. And such holes are not all in one line, but are distributed symmetrically about the screen, care being taken that each zone shall be represented by holes separated radially and also tangentially, corresponding to the pairs of elements in the two astigmatic image surfaces, an arrangement which enables the astigmatism as well as figure to be investigated.

The arrangement of holes actually found useful is shown in Hartmann’s original papers, and also in a very important paper by Plaskett (Ap. J. 25 195) which contains the best account in English of Hartmann’s methods and their application. Now[Pg 214]
[Pg 215] each hole in the screen transmits a pencil of light through the objective at the corresponding point, and each pencil comes to a focus and then diverges, the foci being distributed somewhere in the vicinity of what one may regard as the principal focus, B. For instance in Fig. 167 are shown five pairs of apertures a, a', b, b', etc., in five different zones. Now if a photographic plate be exposed a few inches inside focus as at C each pencil from an aperture in the screen will be represented by a dot on the photograph, at such distance from the axis and from the corresponding dot on the other side of the axis as the respective inclinations of the pencils of light may determine.

Similarly a plate exposed at approximately equal distance on the other side of the general focus, as at D, will show a pattern of dots due to the distribution of the several rays at a point beyond focus. Now if all the pencils from the several apertures met at a common focus in B, the two patterns on the plates C and D would be exactly alike and for equal distance away from focus of exactly the same size. In general the patterns will not exactly correspond, and the differences measured with the micrometer show just how much any ray in question has departed from meeting at an exact common focus with its fellows.

For instance in the cut it will be observed that the rays e and a' focus barely beyond C and by the time they reach D are well spread apart. The relative distance of the dots upon these corresponding plates, with the distance between the plates, shows exactly at what point between C and D these particular rays actually did cross and come to a focus.

Determining this is merely a matter of measuring up similar triangles, for the path of the rays is straight. Similarly inspection will show that the rays d and d' meet a little short of B, and measurement of their respective records on the plates C and D would show the existence of a zone intermediate in focus between the focus of e,e' and the general focus at B. The exact departure of this zone from correct focus can therefore be at once measured.

A little further examination discloses the fact that the outer zone represented by the rays a,b, and a',b' has not quite the same focus at the two extremities of the same diameter of the objective. In other words the lens is a little bit flatter at one end of this diameter than it is at the other, so that the rays here have considerably longer focus than they should, a fault by no means unknown although fortunately not very common.

It will be seen that the variations between the two screen patterns on C and D, together with the difference between them, give accurately the performance of each point of the objective represented by an aperture in the screen. And similar investigations by substantially the same method may be extended to the astigmatic variations, to the general color correction, and to the difference in the aberrations for the several colors. The original papers cited should be consulted for the details of applying this very precise and interesting test.

It gives an invaluable record of the detailed corrections of an objective, and while it is one with which the ordinary observer has little concern there are times when nothing else can give with equal precision the necessary record of performance. There are divers other tests used for one purpose or another in examining objectives and mirrors, but those here described are ample for nearly all practical purposes, and indeed the first two commonly disclose all that it is necessary to know.

Now and then one has to deal with an objective which is unmitigatedly dirty. It can be given a casual preliminary cleaning in the way already mentioned, but sometimes even this will not leave it in condition for testing. Then one must get down to the bottom of things and make a thorough job of it.

The chief point to remember in undertaking this is that the thing which one is cleaning is glass, and very easy to scratch if one rubs dust into it, but quite easy to clean if one is careful. The second thing to be remembered is that once cleaned it must be replaced as it was before and not in some other manner.

The possessor of a dirty objective is generally advised to take it to the maker or some reliable optician. If the maker is handy, or an optician of large experience in dealing with telescope objectives is available, the advice is good, but there is no difficulty whatever in cleaning an objective with the exercise of that ordinary care which the user of a telescope may be reasonably expected to possess.

It is a fussy job, but not difficult, and the best advice as to how to clean a telescope objective is to “tub” it, literally, if beyond the stage where the superficial wiping described is sufficient.

To go about the task one first sets down the objective in its cell on a horizontal surface and removes the screws that hold in the retaining ring, or unscrews the ring itself as the case may be. This leaves the cell and objective with the latter uppermost and free to be taken out. Prepare on a table a pad of anything soft, a little smaller than the objective, topping the pad with soft and clean old cloth; then, raising up the cell at an edge, slip the two thumbs under it and lay the fingers lightly on the outer lens of the objective, then invert the whole affair upon the pad and lift off the cell, leaving the objective on its soft bed.

Before anything else is done the edge of the objective should be marked with a hard lead pencil on the edge of both the component lenses, making two well defined v’s with their points touching. Also, if, as usual, there are three small separators between the edges of the flint and crown lenses, mark the position of each of these 1, 2, 3, with the same pencil.

Forming another convenient pad of something soft, lift off the upper lens, take out the three separators and lay them in order on a sheet of paper without turning them upside down. Mark alongside each, the serial number denoting its position. Then when these spacers, if in good condition, are put back, they will go back in the same place rightside up, and the objective itself will go back into place unchanged.

Now have at hand a wooden or fibre tub or basin which has been thoroughly washed out with soap and water and wiped dry. Half fill it with water slightly lukewarm and with a good mild toilet soap, shaving soap for example, with clean hands and very soft clean cloth, go at one of the lenses and give it a thorough washing. After this it should be rinsed very thoroughly and wiped dry. As to material for wiping, the main thing is that it must be soft and free from dust that will scratch. Old handkerchiefs serve a good turn.

Dr. Brashear years ago in describing this process recommended cheese cloth. The present day material that goes under this name is far from being as soft at the start as it ought to be, and only the best quality of it should be used, and then only after very thorough soaking, rinsing and drying. The very soft towels used for cleaning cut glass, if washed thoroughly clean and kept free from dust, answer perfectly well. The cheese cloth has the advantage of being comparatively cheap so that it can be thrown away after use. Whatever the cloth, it should be kept, after thorough washing and drying, in a closed jar.

Rinsing the lens thoroughly and wiping it clean and dry is the main second stage of cleansing. It sometimes will be found to be badly spotted in a way which this washing will not remove. Sometimes the spotting will yield to alcohol carefully rubbed on with soft absorbent cotton or a bunch of lens paper.

If alcohol fails the condition of the surface is such as to justify trying more strenuous means. Nitric acid of moderate strength rubbed on with a swab of absorbent cotton will sometimes clear up the spotting. If this treatment be used it should be followed up with a 10 per cent solution of pure caustic potash or moderately strong c.p. ammonia and then by very thorough rinsing. Glass will stand without risk cautious application of both acid and alkali, but the former better than the latter.

Then a final rinsing and drying is in order. Many operators use a final washing with alcohol of at least 90 per cent strength which is allowed to evaporate with little or no wiping. Alcohol denatured with methyl alcohol serves well if strong enough but beware denatured alcohol of unknown composition. Others have used petroleum naphtha and things of that sort. At the present time these commercial petroleum products are extremely uncertain in quality, like gasoline, being obtained, Heaven knows how, from the breaking down of heavier petroleum products.

If pure petroleum ether can be obtained it answers quite as well as alcohol, but unless the volatile fluid is pure it may leave streaks. Ordinarily neither has to be used, as after the proper wiping the glass comes perfectly clean. This done the glass can be replaced on the pad whence it came and its mate put through the same process.

Flint glass is more liable to spot than the crown, but the crown is by no means immune against the deterioration of the surface, perhaps incipient devitrification, and during the war many objectives “went blind” from unexplained action of this character. As a rule the soap and water treatment applied with care leaves even a pretty hard looking specimen of objective in fairly good condition except for the scratches which previous users have put upon it.

Then if the spacing pieces, usually of tinfoil, are not torn or corroded they can be put back into place, the one lens superimposed upon the other, and the pair put back into the cell by dropping it gently over them and re-inverting the whole, taking care this time to have soft cloth or lens paper under the fingers. Then the retaining ring can be put into place again and the objective is ready for testing or use as the case may be.

If the spacers are corroded or damaged it may be necessary to replace them with very thin tinfoil cut the same size and shape, leaving however a little extra length to turn down over the edge of the lower lens. They are fastened in place on the extreme edge only by the merest touch of mucilage, shellac or Canada balsam, whichever comes to hand. The one important thing is that the spacers should be entirely free of the sticky material where they lap over the edge of the lens to perform the separation. This lap is generally not over 1/16 of an inch, not enough to show at the outside of the objective when it is in its cell. When the upper lens is lightly pressed down into place, after the gum or shellac is dry, all the projecting portion can be trimmed away with a sharp pen-knife leaving simply the spacers in the appointed places from which the original ones were removed.

Some little space has been given to this matter of cleaning objectives, as in many situations objectives accumulate dirt rather rapidly and it is highly desirable for the user to learn how to perform the simple but careful task of cleansing them.

In ordinary use, when dirt beyond the reach of mere dusting with a camel’s hair brush has stuck itself to the exterior of an objective, a succession of tufts of absorbent cotton or wads of lens paper at first dampened with pure water or alcohol and then followed lightly, after the visible dirt has been gently mopped up, by careful wiping with the same materials, will keep the exterior surface in good condition, the process being just that suggested in the beginning of this chapter as the ordinary cleaning up preparatory to a thorough examination.

The main thing to be avoided in the care of a telescope, aside from rough usage generally, is getting the objective wet and then letting it take its chances of drying. In many climates dew is a very serious enemy and the customary dew cap three or four diameters long, bright on the outside and blackened within, is of very great service in lessening the deposit of dew upon the glass. Also the dew cap keeps out much stray light that might otherwise do mischief by brightening the general field. In fact its function as a light-trap is very important especially if it is materially larger in diameter than the objective and provided with stops.

The finder should be similarly protected, otherwise it will mysteriously go blind in the middle of an evening’s work due to a heavy deposit of moisture on the objective. The effect is an onset of dimness and bad definition which is altogether obnoxious.

As regards the metal parts of a telescope they should be treated like the metal parts of any other machine, while the moving parts require from time to time a little touch of sperm or similar oil like every other surface where friction may occur.

The old fashioned highly polished and lacquered brass tube was practically impossible to keep looking respectably well provided it was really used to any considerable extent. About the most that could be done to it was dusting when dusty, and cautiously and promptly wiping off any condensed moisture. The more modern lacquered tubes require very little care and if they get in really bad condition can be relacquered without much expense or difficulty.

Wooden tubes, occasionally found in old instruments, demand the treatment which is accorded to other highly finished wooden things, occasional rubbing with oil or furniture polish according to the character of the original surface. Painted tubes may occasionally require a fresh coat, which it does not require great skill to administer. If the surface of wooden tripods comes to be in bad shape it needs the oil or polish which would be accorded to other well finished wooden articles.

Mountings are usually painted or lacquered and either surface can be renewed eventually at no great trouble. Bright parts may be lightly touched with oil as an ordinary rust preventive.

Reflecting telescopes are considerably more troublesome to keep in order than refractors owing to the tender nature of the silvered surface. It may remain in good condition with fairly steady use for several years or it may go bad in a few months or a few weeks. The latter is not an unusual figure in telescopes used about a city where smoke is plentiful. The main thing is to prevent the deposit of dew on the mirror, or getting it wet in any other way, for in drying off the drops almost invariably leave spots.

Many schemes have been proposed for the prevention of injury to the mirror surface. A close fitting metal cover, employed whenever the mirror is not in use, has given good results in many places. Where conditions are extreme this is sometimes lined with a layer of dry absorbent cotton coming fairly down upon the mirror surface, and if this muffler is dry, clean, and a little warmer than the mirror when put on, it seems to be fairly effective. Preferably the mirror should be kept, when not in use, at a little higher temperature than the surrounding air so that dew will not tend to deposit upon it.

As to actual protective measures the only thing that seems to be really efficient is a very thin coating of lacquer, first tried by Perot at the Paris Observatory. The author some ten years since took up the problem in protecting some laboratory mirrors against fumes and moisture and found that the highest grade of white lacquer, such as is used for the coating of fine silverware in the trade, answered admirably if diluted with six or eight volumes of the thinner sold with such commercial lacquers. It is best to thin the lacquer to the requisite amount and then filter.

If now a liberal amount of the mixture is poured upon the mirror surface after careful dusting, swished quickly around, and the mirror is then immediately turned up on edge to drain and dry, a very thin layer of lacquer will be left upon it, only a fraction of a wave length thick, so that it shows broad areas of interference colors.

Treated in this way and kept dry the coating will protect the brilliancy of the silver for a good many months even under rather unfavorable circumstances. After trying out the scheme rather thoroughly the treatment was applied to the 24 inch reflector of the Harvard Observatory and has been in use ever since. The author applied the first coating in the spring of 1913, and since that time it has only been necessary to resilver perhaps once in six months as against about as many weeks previously.

The lacquer used in this case was the so-called “Lastina” lacquer made by the Egyptian Lacquer Company of New York, but there are doubtless others of similar grade in the market. It is a collodion lacquer and in recent years it has proved desirable to use as a thinner straight commercial amylacetate rather than the thinner usually provided with the lacquer, perhaps owing to the fact that difficulty of obtaining materials during the war may have caused, as in so many other cases, substitutions which, while perfectly good for the original purpose did not answer so well under the extreme conditions required in preserving telescope mirrors.

The lacquer coating when thinned to the extent here recommended does not apparently in any way deteriorate the definition as some years of regular work at Harvard have shown. Some experimenters have, however, found difficulty, quite certainly owing to using too thick a lacquer. The endurance of a lacquer coating where the mirror is kept free from moisture, and its power to hold the original brilliancy of the surface is very extraordinary.

The writer took out and tested one laboratory mirror coated seven years before, and kept in a dry place, and found the reflecting power still a little above .70, despite the fact that the coating was so dry as to be almost powdery when touched with a tuft of cotton. At the start the mirror had seen some little use unprotected and its reflection coefficient was probably around .80. If the silver coating is thick as it can be conveniently made, on a well coated mirror, the coat of lacquer, when tarnish has begun, can be washed off with amylacetate and tufts of cotton until the surface is practically clear of it, and the silver itself repolished by the ordinary method and relacquered.

There are many silvering processes in use and which one should be chosen for re-silvering a mirror, big or little, is quite largely a matter of individual taste, and more particularly experience. The two most used in this country are those of Dr. Brashear and Mr. Lundin, head of the Alvan Clark Corporation, and both have been thoroughly tried out by these experienced makers of big mirrors.

The two processes differ in several important particulars but both seem to work very successfully. The fundamental thing in using either of them is that the glass surface to be silvered should be chemically clean. The old silver, if a mirror is being resilvered, is removed with strong nitric acid which is very thoroughly rinsed off after every trace of silver has been removed. Sometimes a second treatment with nitric acid may advantageously follow the first with more rinsing. The acid should be followed by a 10 per cent solution of c.p. caustic potash (some operators use c.p. ammonia as easier to clear away) rinsed off with the utmost thoroughness.

On general principles the last rinsing should be with distilled water and the glass surface should not be allowed to dry between this rinsing and starting the silvering process, but the whole mirror should be kept under water until the time for silvering. In Dr. Brashear’s process the following two solutions are made up; first the reducing solution as follows:

• Rock candy, 20 parts by weight.
• Strong nitric acid (spec. gr. 1.22), 1 part.
• Alcohol, 20 parts.
• Distilled water, 200 parts.

This improves by keeping and if this preparation has to be hurried the acid, sugar and distilled water should be boiled together and then the alcohol added after the solution is cooled.

Second, make up the silvering solution in three distinct portions; first the silver solution proper as follows:

• 1. 2 parts silver nitrate. 20 parts distilled water.
• Second, the alkali solution as follows:
• 2. 1? parts c.p. caustic potash. 20 parts distilled water.
• Third, the reserve silver solution as follows:
• 3. ¼ part silver nitrate. 16 parts distilled water.

The working solution of silver is then prepared thus: Gradually add to the silver solution No. 1 the strongest ammonia, slowly and with constant stirring. At first the solution will turn dark brown and then it will gradually clear up. Ammonia should be added only just to the point necessary to clear the solution.

Then add No. 2, the alkali solution. Again the mixture will turn dark brown and must be cautiously cleared once more with ammonia until it is straw colored but clear of precipitate. Finally add No. 3, the reserve solution, very cautiously with stirring until the solution grows darker and begins to show traces of suspended matter which will not stir out. Then filter the whole through absorbent cotton to free it of precipitate and it is ready for use. One is then ready for the actual silvering.

Now there are two ways of working the process, with the mirror face up, or face down. The former is advantageous in allowing better inspection of the surface as it forms, and also it permits the mirror of a telescope to be silvered without removing it from the cell, as was in fact done habitually in case of the big reflector of the Alleghany Observatory where the conditions were such as to demand re-silvering once a month. The solution was kept in motion during the process by rocking the telescope as a whole.

When silvering face up the mirror is made to form the bottom of the silvering vessel, being fitted with a wrapping of strong paraffined or waxed paper or cloth, wound several times around the rim of the mirror and carried up perhaps half the thickness of the mirror to form a retainer for the silvering solution. This band is firmly tied around the edge of the mirror making a water tight joint. Ritchey uses a copper band fitted to the edge of the mirror and drawn tight by screws, and finishes making tight with paraffin and a warm iron.

In silvering face down the mirror is suspended a little distance above the bottom of a shallow dish, preferably of earthen ware, containing the solution. Various means are used for supporting it. Thus cleats across the back cemented on with hard optician’s pitch answer well for small mirrors, and sometimes special provision is made for holding the mirror by the extreme edge in clamps.

Silvering face down is in some respects less convenient but does free the operator from the very serious trouble of the heavy sediment which is deposited from the rather strong silver solution. This is the essential difficulty of the Brashear process in silvering face up. The trouble may be remedied by very gentle swabbing of the surface under the liquid with absorbent cotton, from the time when the silver coating begins fairly to form until it is completed.

The Brashear process is most successfully worked at a temperature between 65° and 70° F. and some experience is required to determine the exact proportion of the reducing solution to be added to the silvering solution. Ritchey advises such quantity of the reducing solution as contains of sugar one-half the total weight of the silver nitrate used. The total amount of solution after mixing should cover the mirror about an inch deep. Too much increases the trouble from sediment and fails to give a clean coating. The requisite quantity of reducing solution is poured into the silvering solution and then immediately, if the mirror is face up, fairly upon it, without draining it of the water under which it has been standing.

If silvering face down the face will have been immersed in a thin layer of distilled water and the mixed solutions are poured into the dish. In either case the solution is rocked and kept moving pretty thoroughly until the process is completed which will take about five minutes. If silvering is continued too long there is likelihood of an inferior whitish outer surface which will not polish well, but short of this point the thicker the coat the better, since a thick coat stands reburnishing where a thin one does not and moreover the thin one may be thin enough to transmit some valuable light.

When the silvering is done the solution should be rapidly poured off, the edging removed or the mirror lifted out of the solution, rinsed off first with tap water and then with distilled, and swabbed gently to clear the remaining sediment. Then the mirror can be set up on edge to dry. A final flowing with alcohol and the use of a fan hastens the process.

In Lundin’s method the initial cleaning process is the same but after the nitric acid has been thoroughly rinsed off the surface is gently but thoroughly rubbed with a saturated solution of tin chloride, applied with a wad of absorbent cotton. After the careful rubbing the tin chloride solution must be washed off with the utmost thoroughness, preferably with moderately warm water. It is just as important to get off the tin chloride completely, as it is to clean completely the mirror surface by its use. Otherwise streaks may be left where the silvering will not take well.

When the job has been properly done one can wet the whole surface with a film of water and it will stay wet even when the surface is slightly tilted. As in the Brashear process the mirror must be kept covered with water. Mr. Lundin always silvers large mirrors face up, and forms the dish by wrapping around the edge of the mirror a strip of bandage cloth soaked in melted beeswax and smoothed off by pulling it while still hot between metal rods to secure even distribution of the wax so as to make a water tight joint. This rim of cloth is tied firmly around the edge of the mirror and the strings then wet to draw them still tighter.

Meanwhile the water should cover the mirror by ¾ of an inch or more. It is to be noted that in the Lundin process ordinary water is usually found just as efficient as distilled water, but it is hardly safe to assume that such is the case, without trying it out on a sample of glass.

There are then prepared two solutions, a silver solution,
2.16 parts silver nitrate (see King, Pop. Ast 30, 93)
100 parts water.
and a reducing solution,
4 parts Merck’s formaldehyde
20 parts water.

This latter quantity is used for each 100 parts of the above silver solution, and the whole quantity made up is determined by the amount of liquid necessary to cover the mirror as just described.

The silver solution is cautiously and completely cleared up by strong ammonia as in the Brashear process. The silver and reducing solutions are then mixed, the water covering the mirror poured quickly off, and the silvering solution immediately poured on. The mirror should then be gently rocked and the silver coating carefully watched as it forms.

As the operation is completed somewhat coarse black grains of sediment will form and when these begin to be in evidence the solution should be poured off, the mirror rinsed in running water, the edging removed while the mirror is still rinsing and finally the sediment very gently swabbed off with wet absorbent cotton. Then the mirror can be set up to dry.

The Lundin process uses a considerably weaker silver solution than the Brashear process, is a good deal more cleanly while in action, and is by experienced workers said to perform best at a materially lower temperature than the Brashear process, with the mirror, however, always slightly warmer than the solution. Some workers have had good results by omitting the tin chloride solution and cleaning up the surface by the more ordinary methods. In the Lundin process the solution is sufficiently clear for the density acquired by the silver coating to be roughly judged by holding an incandescent lamp under the mirror. A good coating should show at most only the faintest possible outline of the filament, even of a gas filled lamp.

Whichever process of silvering is employed, and both work well, the final burnishing of the mirror after it is thoroughly dry is performed in the same way, starting by tying up a very soft ball of absorbent cotton in the softest of chamois skin.

This burnisher is used at first without any addition, simply to smooth and condense the film by going over it with quick, short, and gentle circular strokes until the entire surface has been thoroughly cleaned and begins to show a tendency to take polish. Then a very little of the finest optical rouge should be put on to the same, or better another, rubber, and the mirror gone steadily over in a similar way until it comes to a brilliant polish.

A good deal of care should be taken in performing this operation to avoid the settling of dust upon the surface since scratches will inevitably result. Great pains should also be taken not to take any chance of breathing on the mirror or in any other way getting the surface in the slightest degree damp. Otherwise it will not come to a decent polish.

Numerous other directions for silvering will be found in the literature, and all of them have been successfully worked at one time or another. The fundamental basis of the whole process is less in the particular formula used than in the most scrupulous care in cleaning the mirror and keeping it clean until the silvering is completed. Also a good bit of experience is required to enable one to perform the operation so as to obtain a uniform and dense deposit.