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NOTE A., Page 14.

Rangoon oil is not found in this market. It is a variety of naphtha or petroleum found at Rangoon, in Asia, and contains a variable percentage of solid paraffin. Kerosene with a very little paraffin dissolved in it, would undoubtedly answer the same purpose.

Good sperm oil or tallow (free from salt) will serve quite as well. It is scarcely necessary to caution the reader that all oily matter should be removed from the knife before it is used for cutting sections. This is best effected by dipping it in hot water and carefully wiping it.

NOTE B., Page 15.

Methylated spirit is alcohol which contains a small percentage of methyl. This renders it unfit for use in the manufacture of liquors or even chloroform, but does not injure it as a preservative fluid. The scientific men of Great Britain have the privilege of obtaining this fluid free from duty, and consequently they make very general use of it. In this country the student of science has no such privilege, and must use the more expensive alcohol. This however is but of a piece with other restrictions placed upon scientific pursuits in this country, where men like Marshall Jewell and Hannibal Hamlin have influence enough to pass laws which prohibit the exchange by mail of microscopic specimens, which pass freely in every other civilized country.

True methyl alcohol, or methylated spirits, is not to be had in this country, and therefore whenever this liquid is directed to be used in English books the American microscopist must substitute good alcohol. What is sold for “methyl alcohol” is wood spirit or wood naphtha, and will not serve the purposes of the microscopist, though it answers well enough for spirit lamps, making varnishes, etc.

NOTE C., Page 20.

The method described in the text is generally known as the “free-hand” method, and although it is not generally employed for the production of cabinet specimens or slides that are intended for sale, yet it possesses so many advantages that every student should endeavor to acquire the art of cutting sections without the use of the microtome. For purposes of study such sections are quite equal to the best of those usually found on sale, and for the encouragement of beginners we may add that we have seen sections cut by the free-hand method which for size, thinness and accuracy, excelled any that we have ever seen produced with the microtome. Indeed SchÄffer distinctly affirms that no microtome can equal the skilled hand in the production of thin sections.

NOTE D., Page 22.

The mere existence of a binding screw should be no objection, and where bushings are employed it is sometimes necessary. But as the author well observes, a binding screw cannot be used to secure histological preparations.

NOTE E., Page 22.

A still better system is where the section-knife is held in a frame which slides on the plate so that the edge is kept just above the surface. In this way the edge of the knife is never dulled by grinding on glass or metal. A common mistake in this form, however, is to raise the cutting edge too far above the surface of the plate. This gives rise to great irregularities.

As the size of the sections which it may be desired to cut, varies within wide limits, the best microtomes made in this country are furnished with as many as three tubes or “bushings,” fitting one within the other, and in this way the hole may be partially filled up and thus reduced in size. This enables us to obtain a range of from one inch and a quarter, suitable for large histological preparations, to one-eighth of an inch for such objects as hair sections.

NOTE G., Page 37.

Success in cutting sections depends more upon the excellence of the knife that is used than upon any other one point, and therefore the art of keeping the knife in good order is one which should be acquired by every microscopist, even at the cost of considerable time, study and labor. As a stepping stone to the acquisition of this art, there are certain general principles which must be thoroughly understood before we can hope for success in practice. These principles are simple, and when once clearly stated, quite obvious, but they are frequently overlooked.

Cutting instruments act in two ways—either as simple wedges or as a series of wedges, the latter being generally known as the saw-like action. It is frequently stated in school philosophies that the edges of all cutting instruments are toothed like a saw, and that they act like a saw; this is evidently not the case with many, such as the axe, the chisel, the plane, etc., and it is not even true of several to which it is frequently applied, such as the razor, the penknife, etc. The action can be said to be “saw-like” only when the teeth act as independent wedges and remove a small portion of the substance acted upon, the portion being known as saw-dust. Now in cutting sections of wood with a sharp razor, where but a single stroke is used to carry the cutting edge across a section, say half an inch in diameter, the edge of the razor does not act like a saw; none of the material is removed except the section itself; in other words, there is no saw-dust. That the edge of the knife or razor is not even, is very easily proved by means of the microscope, but it will be found that the notches and projections form a series of lancet-shaped knifelets, each one of which acts just as does the blade of a penknife when drawn over a piece of wood or a quire of paper. Now if we lay a quire of paper on a board and draw the edge of a penknife over it, we will cut down through the paper, not by sawing but by a moving wedge. It is precisely in this way that the little lancet-shaped teeth on the edge of a razor act when the latter is used to cut a section.

If we further examine the edge of the razor, we will find that the bottoms of the notches are comparatively blunt; if therefore we press the razor directly forward we soon press these blunt portions of the general edge against the material to be cut, and the resistance becomes so great that the material is actually crushed or torn. This would not happen if we had an absolutely perfect edge—one without any notches. This being practically unattainable, however, we must endeavor to make up for it by a sawing motion, (though not a sawing action) at the same time seeking to avoid as much as possible those saw-like features which render this motion necessary. We have dwelt at length upon this point because so much has been written comparing the edge of a razor to that of a saw, that some persons actually advise us to give the edges of our knives and razors a saw-like character, under the impression that by so doing they will work better. This is a fallacy; the best work will be obtained from the smoothest and most perfect edge, provided the latter is thin enough.

If the material of which our cutting instruments are made would allow it, that is to say if it was perfectly hard, perfectly rigid and incapable of being crushed or torn apart, the proper shape of a cutting instrument would be a wedge of the most acute form possible. But since the steel which we use is limited in its hardness and its power of resisting crushing and bending influences, the blades of our knives must have a certain strength or thickness, depending upon the character of the material to be cut. For very hard substances the knife must be stout and the edge ground to a comparatively obtuse angle; for moderately soft substances, such as wood, the angle may be more acute, while for very soft tissues the thinnest blade and the sharpest edge are most suitable.

The various angles to which cutting tools of different kinds are ground are shown in the accompanying illustrations, which explain themselves.

In the diagrams just given, the sides of the wedge are carried in straight lines to the very edge. In practice, however, this is never done, there being two strong objections to such a form. In the first place if the sides were as shown in the figures, every time the tool was ground or set, the entire side would have to be ground off, and this would involve great labor. But another difficulty, quite as great, would occur in the use of the instrument, for since the wedge keeps growing constantly and proportionately thicker, the friction between the sides of the wedge and the material would rapidly become very great. Therefore most cutting edges are ground to a second angle, much more obtuse than the first, and it is this second angle which forms the true cutting edge. And it is obvious that the mass of material required to carry out the ultimate angle in any case is wholly unnecessary, for if the tool be strong enough at a b (Fig. 2) it will certainly be strong enough at c d. An extreme illustration of this principle is shown in the cold chisel used for cutting iron, the edge of which is shown in Fig. 2, and the angle of 80° to which it is ground is carried out by dotted lines. The angle which the sides of the chisel (not the sides of the extreme cutting edge however) make with each other is 25°.

There are two methods by which this change of angle may be made, one of which is shown in the cold chisel just figured, and the other in the razor of which a section is shown in figures 3 and 4. In the case of the cold chisel it will be seen that the tool is first formed to a thin straight wedge which is afterwards changed to one that is much more blunt. This answers very well where the ultimate angle is comparatively large or blunt, as is the case with penknives, table-knives, carving-knives, etc. But where the ultimate angle is very small this plan does not answer well, and the method shown in Fig. 3 is generally adopted. Here the ultimate angle is such that lines touching the extreme edge and the back of the blade are perfectly straight and form the actual cutting angle. In this case, therefore, the relief is obtained by hollowing out the sides of the blade, and this is done to various extents, the extremes being shown in figures 3 and 4. Fig. 3 shows a section of a razor ground on a stone 12 inches in diameter, which is as large as is generally used for this purpose. Fig. 4 is a section of a razor ground on a four-inch stone—the smallest in general use. This method of changing the angle is of course substantially the same as the first, merely differing in the mechanical device used, but it affords this important advantage that in the subsequent honing and stropping processes the back of the razor forms a perfect guide by which the ultimate angle may be determined. This is not the case with the cold chisel or the carving-knife, in both of which cases the eye and hand alone determine the cutting angle, which is therefore apt to become irregular or even rounded—the worst form of all. This will be more easily understood from the following engravings where A, Fig. 5, shows a penknife blade, as applied to an oilstone for the purpose of giving it the final edge. The angle here shown is considerably greater than that generally used, but the principle is the same. It is evident that if, in moving the blade back and forth on the stone we allow it to rock or change the angle which the blade makes with the stone’s surface, the edge will become rounded as shown in Fig. 6—a form which for delicate work is useless.

If the blade be laid down flat as at B, Fig. 5, the labor of wearing away the superfluous metal will be enormous. But if the blade be hollow as Fig. 7, (where the hollow is, for clearness of illustration, slightly exaggerated) then the ultimate angle may be formed quickly and accurately, there being no danger of the angles being changed on account of want of skill on the part of the operator.

In some cases one side is left quite flat and the other side is ground to two or sometimes three angles, as in the carpenter’s chisel, a section of which is shown in Fig. 8, where a a shows the angle made by the body of the blade; b b the angle of the bevel formed by the grindstone, and c c the angle of the cutting edge which is formed by the oilstone. In this case the flat side A B forms a straight guide and enables the workman, in cutting, to make straight, clean work. In sharpening such a tool on the oilstone the flat side is always laid flat on the oilstone and merely smoothed off,^[19] and the skill of the workman is chiefly shown in the accuracy with which he forms the angle c B c. The dexterity with which skilled workmen can effect this by the hand alone is astonishing, but in the hands of those who have had but little practice the edge infallibly becomes rounded as shown in Fig. 6. Therefore wherever a knife with a flat side is used, no attempt should ever be made to grind this flat side on the hone. Such an attempt will almost always result in an edge so irregular that it will be almost impossible to cut a good section with it. But if the under side of the knife be left truly flat, we will have the very best tool that can be had for use with the microtome. Unfortunately, however, it is very difficult to get a knife with a truly flat surface, that is, one that is perfectly “out of wind” as mechanics say. As a general rule the flat sides of knives are not true planes but irregularly curved surfaces; consequently when laid on a plane surface, such as the table of a microtome, they rock, and the edge is in contact with the table at some points and separated from it at others, and the points which are in contact change as the knife is moved diagonally across the table. This makes the section irregular and worthless. But a really good knife with a perfectly true flat side is such a valuable tool that a good deal of effort may be profitably expended in getting it, and when once obtained never let the flat side be touched by a grindstone, or more than touched by a hone. It is not absolutely necessary, however, that the sides of a delicate knife should be curved, provided the knife is used for cutting very soft materials. An angle of 10° or even 5° gives a pretty stout blade when carried back three-quarters of an inch, as may be seen by examining Fig. 1, and the guiding action of the very thick back may be obtained by means of an artificial guide, applied like the brass backs of tenon saws. Such an arrangement is shown in Fig. 9, and has been found very effectual.

When the knife is to be honed, the back is applied and fastened by means of two or three set screws. When laid on the hone, the edge of the knife and the lower surface of the back form the guide and regulate the cutting angle. In this way we can use a broad, thin blade and yet secure great accuracy in honing it. Such a blade has this advantage also, that it is not so liable to be rounded and thus injured by stropping as one in which the relief is obtained by hollowing out the sides.

Such, then, are the general principles which govern the construction of cutting tools, including the section knives used by microscopists. We will now give a few practical directions for selecting a knife and putting and keeping it in order.

In selecting a knife for cutting sections, regard must be had to the texture of the material that is to be cut. To attempt to cut delicate sections of soft tissues with the stout knives which are suitable for cutting sections of woody fibre would be to destroy the sections, while to reverse the operation and cut sections of wood with thin, delicate knives would result in the ruin of the knife. We have seen a most excellent knife seriously injured by an attempt to cut material that was too hard for it. The knife was very thin, and had proved most excellent in cutting sections of such material as kidney, liver, etc. An attempt was made, against our protest, to cut a section of an apple-shoot, the wood of which was mature. Before the knife had gone half way across, it bent, dug into the wood and broke, leaving an ugly gap in the middle. Experienced section-cutters know this very well, but young microscopists are not so familiar with the facts just stated, and the point is too important to be overlooked. Those, therefore, who devote themselves to microscopical studies, or who expect to make sections of materials of several kinds, differing in hardness, etc., must provide themselves with knives of different degrees of strength.

For common work, good razors are as good as anything, provided they can be obtained with straight edges. Where razors are not suitable, recourse must be had to the surgical instrument maker, though we are sorry to say that there are but few in this country that know how to forge, temper and grind a decent knife. Most of our dealers in instruments do not make the instruments they sell; they import the goods they sell with their names stamped on, and thus get a reputation as manufacturers; a special order they are unable to fill respectably. There are some exceptions, but of the majority of dealers what we have written above is true.

The points which specially demand attention in a knife for cutting sections are these: 1. Quality of the steel used; 2. Temper; 3. Form of the blade.

Of the quality of the material of which a knife is made, nothing can be determined except by actual trial. The old tests of staining with acids, examining with the microscope, etc., are worthless, or at least too crude and uncertain to be of any practical value. Color changes with the degree of polish that is produced, and, in short, there is no reliable guide. The purchaser must depend entirely upon the reputation of the manufacturer. There is plenty of good steel to be found; the trouble lies with the cutlers. They are careless and in haste, and as a consequence they burn the steel or fail to work it sufficiently, and the result is a useless tool.

The steel may be of the very best quality, however, and well forged, and yet the knife may fail from being badly tempered—too soft or too hard. If too soft, the edge is soon dulled; the knife requires to be frequently honed, and the time wasted in keeping it in order is a serious drawback. If too hard, it is impossible to give it a keen edge, for the metal crumbles away as soon as it is honed or stropped very thin, and the edge becomes ragged and dull. Good steel, well forged, may be so tempered that it will neither crumble nor become rapidly dull.

Much may be learned on this point from careful inspection of the edge, and trial on a piece of horn, such as an old razor handle. When drawn over a clean piece of horn once or twice the edge of a soft knife is completely dulled; if well-tempered it should scarcely lose its keenness. Again, when laid flat on the thumb nail and pressed, the edge ought to bend up without breaking or crumbling, and at once, when the pressure is removed, resume its original shape. The extent to which extreme hardness and durability, or toughness, may be combined, is well shown in the famous Toledo sword-blades. One of these blades will shear through an iron nail without having its edge perceptibly dulled, and yet so tough and springy is it that it may be coiled up into a hoop of several folds without breaking or receiving any permanent set. If we could only get section knives of stuff like this, it would be a pleasure to work with them.

After all, however, the only test of these two points, material and temper, is a fair trial in actual practice. Of the form of the blade, however, it is easy to judge, and there are a few points which are frequently overlooked and which give rise to errors that are attributed to other causes.

In the text we are told that “It is essentially necessary that the back and edge of the blade be strictly parallel to each other.” The author undoubtedly knew what the correct form should be, but the definition he has given is not a correct mathematical statement of the conditions involved. These are as follows:

The edge must form a straight line, and both the edge and the under side of the back must lie in the same plane.

Let us consider these conditions somewhat carefully.

Since the hole in which the material to be cut is imbedded, is always circular, and since the blade is generally pressed forcibly against the table of the microtome, it is obvious that if the edge of the knife be curved it will sink into the hole to a greater or less depth according as it is further from the centre of the hole or nearer to it. To explain this let us refer to the greatly exaggerated drawing given in figure 10 where the edge of the knife, a, b, is shown in two different positions covering the hole of the microtome. The least degree of reflection will show that as the knife passes towards the centre from a b to a' b', it will have a tendency to turn on the back as on a hinge, and the middle of the edge will sink deeper into the hole as it approaches the centre of the latter. The section will therefore vary in thickness and prove useless. This difficulty can be obviated only by the use of a straight edge like e f, which will always lie in the same plane.

If the knife were perfectly rigid, and the curved edge and back lay entirely in the same plane, this difficulty would not occur to any great extent. But since a curved edge rarely lies wholly in the plane of the back of the knife, and since the blades of most knives, especially those thin ones used for cutting sections of soft tissues, yield a little to pressure, and since this pressure is never exactly the same, it is impossible to prevent irregularities from occurring.

While it is not necessary that the back of the knife and the cutting edge should be parallel to each other, it is absolutely necessary that they be in the same plane, or, as mechanics say,—“out of wind.” If this condition be not observed, the blade will rock on the microtome table as it passes over it, and irregularity in thickness of section will result.

A slight knowledge of geometry will enable any one to see that these statements are strictly accurate.

As regards other points in the form of the knife, we refer to what has been previously said when discussing the general principles which should govern the construction of cutting tools.

We now proceed to give a few practical directions for sharpening knives and razors and keeping them in order.

Section knives, during the process of sharpening, are subjected to three distinct processes: 1, Grinding; 2, Honing; 3, Stropping.

Grinding is one of the most important, though it is probable that it will rarely be undertaken by the microscopist himself. More knives and razors are spoiled in grinding than in any other way. We have now in our possession an excellent knife, which in an evil hour we entrusted to a New York cutler, and received it back utterly ruined, the temper being entirely taken out of the blades! Some of our readers may wonder at this, but unfortunately it is too true. The cutler, to save time and trouble, too frequently holds the blade against the stone with such force that it becomes over-heated, especially when he is a little careless and allows the supply of water to fall short.^[20] The owner does not find this out until he discovers that the knife is ruined, because nothing is more easy than to cool the blade and grind off the tell-tale blue spots. When charged with his rascality, the cutler always denies it and lays the blame on a “soft spot in the steel”—a miserable subterfuge—too transparent to deceive the least experienced.

The only protection against this is either to entrust the knife to a man of known carefulness and integrity, or to stand over him while he does it. If the knife be ground under the owner’s eye, no fear need be entertained, because it is easy to insist upon the use of plenty of water.

The grindstone used for section knives should be of fine grain, and it should run true and be very straight across the face, otherwise it will be impossible to grind the knife true, and this, as we have seen, is a necessity.

Every microscopist that expects to do much at cutting sections must learn to hone his own knife or razor. Of the various hones in use the famous Turkey oilstone is said to be altogether the best. It is, however, very difficult to get it genuine and in sufficiently large pieces. We therefore use the German hone-slate, a softer, yellow stone of wonderful fineness and cutting power. The ordinary whetstones are altogether too coarse, and as for artificial stones and strops, they may do for coarse work and common razors for scraping, but they will not do for knives used for section-cutting. If any of our readers should be so fortunate as to possess a good, old-fashioned Turkey oilstone with a fair sized surface, let them think much of it. It is invaluable. Years ago, when working on some investigations that required the use of very sharp knives, we were offered, by an old English carpenter, the use of a real Turkey stone. We have never since found anything that did its work so quickly and so well.

To merely mention the substitutes for Turkey oilstone would fill some pages. We therefore confine ourselves to the German hone. It is obtained from the slate mountains in the neighborhood of Ratisbon, where it occurs in the form of a yellow vein running vertically into the blue slate, sometimes not more than an inch in thickness, and varying to twelve to eighteen inches. After being quarried it is sawed into thin slabs, which are usually cemented to a similar slab of the blue slate as a support, and the combined stones are then set in a wooden frame and fitted with a cover, also of wood. Unlike the Turkey oilstone, the Arkansas stone, and some others, the German hone is soft and easily scratched and worn. It must therefore be carefully preserved, and as the flatness of the hone is an important point, it should never be used for anything but section knives.

Having purchased a hone, the first thing to do is to see that it is perfectly true; that is, that its surface is a perfect plane. If hollow or twisted, it is useless until made straight. It may be tested by means of two steel or wood straight-edges, and the method of doing this, which is very simple but difficult to describe, the microscopist can learn from any cabinet maker or carpenter. If the hone be not true it must be made so, and this is best done by grinding it with emery on a slab of marble or, better still, a plate of cast iron which has been planed true. (Waste castings with planed surfaces may be found in most machine shops, and may be either borrowed or purchased for a small sum.) If the hone is very uneven, coarse emery may be used at first; then finer must be taken, and so on until the surface of the hone is not only true but very smooth. This involves considerable labor, but it cannot be avoided if a true edge as well as a sharp one is desired, and hence our advice to use the hone for nothing but section knives. Accuracy in a penknife or a razor for shaving is of no consequence; in a section-knife it is a sine qua non.

The surface of the hone must not only be true and smooth, it must also be clean and free from dust and grit, a single particle of which may spoil the work of hours. It should therefore, when not in use, be kept constantly covered. It is always used with oil, and this oil should be of a kind that does not readily dry up. Good sperm is excellent, and so is purified neat’s-foot oil. Avoid kerosene, soap and water, and similar fluids, which are so frequently recommended, especially under the guise of new discoveries. They have all been tried and found wanting. After use, the stone should be carefully wiped clean, fresh oil being applied when it is next used. It must never be used without a liberal supply of oil, otherwise it will become glazed and will no longer act on the steel.

The hone being in good order, the blade is laid flat on it and moved over its surface with circular sweeps. The method of doing this is easily acquired, and is best learned by watching an expert cutler at work. If we could only hit it, the perfection of this operation would be to wear down the sides of the blade until they just met; they would then form a keen and perfect edge. Unfortunately we cannot always hit this point exactly; we are apt to overdo the matter just a little, and then comes up the bÊte noir of beginners—the “wire edge.” This is caused by the metal of the blade bending away from the hone as soon as it becomes of a certain degree of thinness. Once it does this it cannot be worn off by the hone, and the only way to remove it is to draw the edge of the knife or razor across a piece of horn or hard wood. After removing the wire edge in this way, give the blade one or two sweeps on the hone and then strop it. If the blade be very soft, the wire edge comes very quickly and easily, and is very hard to get rid of. On very hard blades the wire edge is not so apt to make its appearance. It comes, however, on very excellent blades. It may be avoided in a measure by driving the blade edge-first over the hone, and indeed the blade should always be sharpened most in this way. And since the backward and forward strokes are always nearly equal, as regards space passed over, this is accomplished by exerting a greater pressure on the forward than on the backward stroke.

This wire edge is a nasty thing when it breaks off on the hone. Unless removed it will very speedily ruin a fine knife, therefore look out for it and wipe off the hone carefully if you have any suspicion that particles of steel have broken off the blade and got on to it.

In honing, as in everything else, however, nothing but practice will impart skill, and he who intends to become an expert should practice on a few old razors, grinding, honing and stropping them himself, until he has acquired the art of giving them an edge far keener than most of the razors ordinarily used for shaving.

The last operation to which the knife is subjected is that of stropping, and as the blade should be stropped after every section, it becomes important that our tools should be good and that we should know how to use them. By having two or three knives we can always avoid grinding and honing the blades ourselves, for it is then easy to hire the work done, and half a dozen knives will last a careful worker a long time with merely the aid of a good strop. But the strop he must use himself. Fortunately no great skill is required in stropping, but the strop must be a good one and is best home-made.

Our readers have doubtless seen the “Cheap John” strop-vendors take a rough table knife, strop it on one of their “patent” strops and cut a hair with it. Nevertheless such a knife, sharpened in that way, would not cut good sections, and such a strop is not just the thing for giving the finishing touch to a section-knife, even though the label does say that it is “intended for surgical instruments.”^[21]

The strop should never be used with the intention of removing metal so as to thin the edge of the knife. This is the proper function of the hone, and those who depend most on the hone and least on the strop, will always succeed best. The strop is used in the first place for smoothing off the edge, and in the second for removing that incipient dullness which always comes from even the slightest use. It is true that in the latter case the strop trespasses somewhat on the functions of the hone, but only to a very slight extent, and stropping should never be carried so far as to require more than a very few strokes.

The best material for a strop is good calf-skin, well tanned, and firmly glued to a strip of wood with the hair side out. The leather should be hard, otherwise it will rapidly round the edge of the knife. The way in which this occurs is readily understood from figures 11 and 12. In Fig. 12 the leather is seen to rise up behind the edge of the blade as the latter passes over it, so that instead of two plane facets meeting at an angle of from 15° to 20° as left from the hone, (Fig. 12) the edge has become considerably rounded and the ultimate angle of the cutting edge is nearly or quite doubled. The hardest and firmest leather should therefore be chosen, avoiding of course any leather that has been made hard and unpliable by the action of water or other agents. Two sides will be found enough for a strop. On one the leather should be kept clean, while the other should be thoroughly impregnated with fine grained, but very hard rouge or crocus.^[22] Rouge is an artificial oxide of iron prepared by exposing sulphate of iron to heat. The hardness of the resulting powder depends upon the temperature to which it has been exposed, and this temperature is very well indicated by the color which the rouge assumes. Bright red or crimson rouge is soft and will not cut steel; hard rouge, suitable for polishing steel, is purple in color, and this quality should therefore be chosen. Great care should be taken to see that it is free from gritty particles, and it should be well rubbed into the leather in a dry state. All mixtures of grease and oil with abrasive substances should be eschewed. If the leather be of good texture and the rouge hard and fine, a very few strokes will suffice to impart the last degree of smoothness and keenness to the blade.

Since grit and dust would soon spoil the strop, it should be kept carefully covered and protected. The rouge will require occasional renewing, which may be done by sprinkling a little lightly over the surface and rubbing it in with the finger. Consequently we find that the best and most convenient holder for rouge is a bottle with some fine but porous fabric tied over the mouth of it, forming as it were a sort of dredge. Over the bottle mouth and its cover should be tied a paper cap to keep out dust.

We have thus endeavored to give, in as simple a manner as possible, such information as will guide the microscopist to the acquisition of skill in this most important but generally despised department of his art. We are told by the biographer of Swammerdam that a great deal of his success was to be attributed to his skill in sharpening his fine knives and scissors. The same is also stated of other noted workers, and yet none give even the slightest directions to the novice who desires to follow in their footsteps. There is not to our knowledge a single book in the English language which treats fully of this subject, with perhaps the exception of Holtzapffel’s work on “Mechanical Manipulation,” and this has long been out of print. As previously stated, however, it is only by practice that the necessary finger-skill can be acquired, but this skill is well worth acquiring at any cost. The man who depends upon cutlers and knife grinders will never make satisfactory progress.

NOTE H.

We are inclined to believe that the difficulty of enclosing glycerine does not arise from the dissolving action of this liquid, so much as from the great adhesiveness which exists between it and glass. Oily cements are of course attacked by glycerine, but shellac and several other gums are insoluble, and drying oils, if well oxidized are also insoluble. Where cells are used they must be first firmly attached to the clean slide, and the cement should also be brought into direct contact with the clean cover. Glycerine does not evaporate at ordinary temperatures, and consequently wherever the slide or cover is soiled with it, no cement will ever adhere until the glycerine has been wiped off. There is no prospect of its drying.

NOTE I. Page 48.

The solution here referred to, and which is known as Kleinenberg’s solution, is made in the following way:

(1.) Make a saturated solution of crystallized calcium chloride in 70 per cent. alcohol, and add alum to saturation. (2.) Make also a saturated solution of alum in 70 per cent. alcohol. Add (1) to (2) in the proportion of 1:8. To the mixture add a few drops of a saturated solution of hÆmatoxylin in absolute alcohol.

This solution may be used in very many cases for staining sections, in place of the ordinary watery solution of logwood alum. It may, if required, be diluted with the mixture of 1 and 2. The stained sections are placed at once in strong spirit.