The Romance of the Microscope / An interesting description of its uses in all branches of science, industry, agriculture, and in the detection of crime, with a short account of its origin, history, and development

CHAPTER IV THE COMPOUND MICROSCOPE

In our chapters, dealing with, the history of the Microscope, we attempted to trace the gradual development of the compound instrument from the simple lens; we stated that the latter, in a crude form, had been known and used from very early times and that the former developed side by side with the telescope. We have also said a few words in Chapter III. concerning light for the reason that the microscope can be better understood and used more efficiently when we are acquainted with the phenomena due to light. The simple lens, sold under the name of pocket magnifier, in its cheapest form consists of a double convex lens, that is to say, a lens with two outwardly curved surfaces. Better quality pocket magnifiers consist of two or more lenses, which may be either double convex; plano-convex, i.e., with one surface perfectly flat and the other outwardly curved, or they may be constructed of a combination of double convex and plano-concave lenses, such as were described on p. .

The object of both the simple and compound microscope is to make objects appear larger than they do to the naked eye. When we buy our pocket lens we shall find that these little instruments are constructed to give different degrees of enlargement, some make objects appear five times larger than they do to the naked eye, some ten, some fifteen and some twenty times larger. Twenty times is about the limit of magnification for the ordinary pocket lens. If we are observant we shall notice something else—the greater the magnification the nearer we must hold the lens to our object. Within certain limits, this is not a very serious matter, but a point is reached where we must hold our lens so near to the object that we cannot see it, and that is why we cannot obtain very great enlargement with a pocket lens. Despite this fact, as we read in our opening chapter, some very wonderful discoveries have been made with these simple microscopes.

Now we wish to show how a compound microscope works and, having done so, to explain the uses of its various parts. We shall consider the lenses of the instrument to be double convex; we do this for the sake of simplicity. Even in the cheapest compound microscopes of to-day simple convex lenses are never used, for the reason we explained in our last chapter. To understand the course of the light rays passing through our microscope, however, we may look upon the lenses as being merely double convex.

Let us try a simple experiment first of all. For the purpose we require two double convex lenses, one capable of magnifying more than the other, a sheet of paper and a candle. We must darken the room in which we make the experiment and, having lighted the candle, we may proceed to make a compound microscope, for that is really what we are about to do. Taking the lens which gives the greatest magnification, we look through it till we can see a clearly defined image of the lighted candle, then we fix the lens at that spot, so that, during the rest of our experiment, the candle and lens remain at the same distance from one another. Now we put the piece of paper as nearly as we can in the position of our eye, moving it nearer or further from the lens till we have a perfectly clear image of the candle thrown upon it. The first thing to strike us is that the image is upside down; it is known as a real, inverted image. Real because it can be thrown upon a screen and inverted—well because it is upside down. There are some images, as we shall learn in a moment, which can be seen but which cannot be thrown upon a screen: they are called virtual images.

Having fixed our sheet of paper in position, we take our second lens, focus it sharply upon the back of the sheet of paper, being careful to keep the centres of the two lenses as far as possible in a straight line with one another. Having obtained a sharp image we remove the paper and gradually advance our second lens towards the first. We soon reach a point where we have a very much larger image of the candle than the first lens gave us; we must fix our second lens at this point for we now have a compound microscope, a very crude one certainly and without the trimmings which make the microscope so useful. Before we proceed to explain what has happened to the light rays we must take our paper screen once more and place it as near as possible to the spot where our eye was situated when we saw the second image. We shall find that, however much we may move our screen to or from the second lens we can never manage to obtain an image upon it for the reason that this second image is virtual, but unlike the first image it is not inverted.

One or two diagrams will help to explain our experiment and, instead of the lighted candle, we will suppose that our object is an arrow—it is easier to draw and serves just as well. The magnification of the object by our first lens may be represented by the diagram below, where AA is the lens, CD the object and D'C' its image.

The arrow C'D' shows the point at which we placed our screen, and as our diagram shows, the image is magnified and inverted.

Our second lens, we remember, was focussed on the back of the paper, placed at C'D'; for practical purposes we may ignore the thickness of the paper and say that it was focussed on the image C'D'. Had we left it at that, the further course of the rays through the second lens would be represented by a replica of the diagram we have just given. But, in our experiment, we moved the second lens nearer and nearer to C'D' till we obtained a clear much magnified erect image of C'D', let us call this second image CD, and represent the course of the light rays by a diagram.

We may well ask, why did the lens AA, our first lens, form a real image whilst the second lens BB, which is precisely similar to AA, except that its magnifying power is not so great, form a virtual image? The formation of a real or a virtual image is nothing to do with magnification, so we repeat—why do two similar lenses form different kinds of images? Let us refresh our memories with the remarks concerning the principal focus of lenses in the last chapter, then we may try another experiment. The principle focus of a double convex lens, we remember, is the point to which parallel rays of light converge, after passing through the lens. If now our object is further away from the lens than its principal focus, a state of affairs that existed in the case of our lens AA and the object CD, we obtain a magnified, real but inverted image; if, on the other hand, using the same lens if we wish, the object is nearer to the lens than its principal focus, we obtain a magnified virtual and erect image. The form of image then depends on the relative positions of lens and object and not on the magnifying powers of the former.

After this digression, we will see what happens when we combine the diagram showing the real, inverted image, formed by the lens AA with the virtual erect image, formed by the lens BB. In reality we will draw a diagram showing the path of the light rays through our compound microscope.

We have used the same lettering as in our previous diagrams and we see that, also as before, a real, inverted image C'D' of the object CD is formed by the lens AA and a virtual, erect image CD of the image C'D' is formed by the lens BB, the object CD being further from the lens AA than its principal focus and the image C'D' being nearer to the lens BB than its principal focus. One very important point we must notice before we leave the diagram. We have mentioned several times that the image formed by BB is erect and so it is, but it is an erect image of an already inverted image, so that the final image of CD, as seen by the eye E is inverted. The fact that objects viewed through the microscope appear upside down is puzzling at first. To all intents our two double convex lenses represent a compound microscope; actually, they should be fixed at either end of a tube, blackened on the inside. The lens AA, nearest to the object, would then be known as the objective and the lens BB nearest to the observer’s eye would be known as the ocular or, more commonly the eyepiece. There are, of course, very many refinements, designed to make the instrument capable of performing the most accurate work, and needless to say these simple lenses would neither give very great magnification nor any clear images. Let us describe a more refined compound microscope than the one we constructed in our darkened room. The optical parts, that is to say the lenses, are the most important parts of every microscope, upon their qualities depend the degree of efficiency of the instrument; the metal portions, known collectively as the stand, contribute to the easier, smoother working of the microscope.

By the courtesy of Messrs. F. Davidson & Co.

The Head of a Dog Flea

No wonder the flea is an annoying creature. As the plate shows, it is armed with knives, lances and saws, all designed to injure the skin of its victim.

The stand must claim our attention first. The base of the instrument, called the foot, is usually either three-legged or horse-shoe shaped; whatever its form it should be heavy, for only thus can the microscope be steady, and steadiness is essential in all microscopic work. At the top of the foot there is a joint, in order that all the other parts of the stand may be inclined at any angle, from the vertical to the horizontal. Just above the joint is a bent arm of brass, to the forward end of which a brass tube is affixed. This tube is designed to hold the lenses, the objective at its lower end, the eyepiece at its upper end. The tube is always blackened inside; were this not the case, light passing through the objective would be reflected in all directions from the sides of the tube and a clear image of the object could never be obtained. The tubes of microscopes vary in length according to their country of origin; English and American tubes are ten inches long, those of continental make vary from a little more than six inches to rather more than seven inches in length.

Affixed to the lower end of the bent arm of brass, mentioned above, is a flat metal plate, known as the stage; at its centre, there is a circular hole through which rays of light pass to illuminate objects placed upon it. Below the stage, at the edge nearest to the foot, there is a metal peg, over which fits a tube to which a mirror is attached by a moveable joint. The mirror reflects light rays through the opening in the stage. The tube, holding it, can be slipped up and down the peg under the stage, thereby bringing it nearer to or further from the object and so altering the intensity of the reflected light, as we shall explain in a moment. Owing to its moveable joint, it is possible to swing the mirror to the right or left, so that the reflected light rays do not pass directly through the object on the stage, but strike it on one side or the other, thereby giving what is known as oblique illumination.

The cheapest forms of compound microscopes have all the parts we have mentioned, and focussing is carried out by sliding the tube, with its objective and eyepiece, up and down within its holder, in order to bring the objective further from or nearer to the object.

In more expensive instruments there are further refinements, in fact, on some of the very costly present-day instruments, there are so many appendages and appurtenances that it is doubtful whether some of them are not more of a hindrance than a help, at any rate they increase the possibility of trouble by their liability to get out of order. Such microscopes are only of use to very expert workers; there are, however, a good many additional features to be found on quite moderate-priced instruments, features which are a great help to the microscopist.

It is obvious that we cannot attain any degree of accuracy in focussing, especially with high magnifications, when we must perforce raise or lower the tube by hand. To obviate this difficulty, most microscopes are provided with mechanism known as a coarse adjustment; it consists of milled screws at either end of a metal rod; in the centre of the rod there is a little cog-wheel which engages with a row of notches on the tube. By turning the milled screws slightly in either direction, we can impart a considerable upward or downward movement to the tube carrying the objective and focussing at once becomes a more simple matter. The coarse adjustment is only useful for examining objects with a low magnification; if we use it when objects are being highly magnified we run the risk of screwing our objective down upon our object, to the certain destruction of the latter and the probable injury of the former. To obviate such a catastrophe, most of the better class microscopes are also provided with a fine adjustment. By means of this adjustment, which externally takes the form of a single milled screw, a considerable turn of the screw in either direction only imparts a very slight upward or downward movement to the microscope tube. In the best instruments, movements of as little as one hundredth part of a millimetre may be imparted to the tube by the fine adjustment and, seeing that there are about twenty-five and a half millimetres to the inch, it is obvious that a good fine adjustment is very delicate and, being so, must be treated with care. The fine adjustment is used to supplement coarse adjustment in the final focussing, when using high magnifications.

A few words may be devoted to the mirror, for on its intelligent use much depends. Usually we shall find that it is plano-concave, that is to say, flat on one side and hollowed out on the other. The use of the mirror, as we have mentioned already, is to reflect rays of light through the opening of the stage on to the object we desire to examine. Both mirrors will reflect parallel rays of light to a point, just as a double convex lens will so direct them from their course that they meet at a point. The concave mirror gives the more powerful illumination, because it reflects more light rays than a flat mirror of the same diameter.

We have mentioned that, to obtain full advantage from the mirror it should be capable of movement to and from the stage. When we desire strong illumination we arrange the mirror so that its reflected rays meet at a point coinciding with our object. Should less intense illumination be required, we slide the mirror nearer to the stage, and of course nearer to our object, so that the reflected rays meet at a point above the object.

The two diagrams, given below, show the path of the rays of light, where O is the object, and a trial with our microscope will soon show which position gives the more powerful illumination.

For high-power work, such as bacteriology or even the examination of sections of plants, etc., even the best concave mirror will not give a sufficiently powerful illumination; accordingly an instrument, known as the condenser, is fixed below the stage, between the mirror and the object. The condenser, as its name implies, condenses the rays of light reflected to it by the mirror. It consists of a series of lenses so arranged that they will throw a very powerful cone of light. Provision is made for focussing the rays from the condenser on to the object.

Sometimes, for special forms of illumination, it is necessary to cut off some of the rays of light passing through the condenser. It may be that we desire to dispense with the outer rays of the cone of light or, when delicate details are being studied, we may wish to impede the central rays. In either case diaphragms, popularly called “stops” are used. Our diagrams show A the outer rays of a cone of light cut off and B the central rays similarly treated.

In old pattern microscopes and in many instruments not provided with condensers, the diaphragm used for the purpose of cutting off the outer rays of the cone of light, consists of a blackened circular metal plate, perforated with a number of different sized circular holes. This plate is fixed below the stage in such a manner that, as it is revolved, holes of various diameters are brought one by one within the cone of light. It need hardy be remarked that the smaller the hole in the diaphragm the more light is cut off and the less reaches the object. In more modern instruments and in practically all which are fitted with a condenser, an Iris diaphragm is fitted. A diaphragm of this nature consists of a number of thin, blackened, metal leaves, fastened to a metal ring in such a manner that, when the ring is revolved, the leaves close together, making the opening in the centre smaller and smaller. The Iris diaphragm has many advantages over the old perforated metal plate. At will, we can have any opening from full to the merest pin-point or we can cut off the light rays altogether, should we wish to do so; we are not confined to a definite number of stops. As we cut off these outer rays of light we shall find that, up to a certain point, though the illumination becomes less and less the object becomes more and more clear, or, to use the correct expression, its definition is improved.

When it is necessary to cut off some of the central rays of the light cone, either a circle of glass with an opaque centre is dropped into a metal holder below the stage, or a circular metal plate, held in the centre of a metal ring by three arms, is used in the same manner.

The effect of cutting off the central rays of the light cone is, of course, to reduce the illumination and to show up delicate detail to advantage. No direct rays of light reach the objective, such as do pass into the microscope are all diffused from the edges of the object.

We have already mentioned that the optical parts of the compound microscope are of greater importance than what may be termed the mechanical portions and the objectives are more important than the eyepieces. Better results can always be obtained with a good, high-power objective and a low-power eyepiece, than with an inferior objective and a good quality eyepiece. The merits of the eyepiece, however great, will not be adequate compensation for the failings of the objective. Modern objectives are composed of several lenses and of a combination of flint and crown glass, as we explained in our last chapter. They are so designed that they can be screwed into the lower part of the microscope tube. The focal length of each objective is, or should be, marked upon it; as a general rule, however, it may be taken that the smaller the lower lens, the shorter its focal length and therefore the greater its magnifying power.

The form of eyepiece most usually met with is known as Huyghen’s. It consists of two plano-convex lenses, with their flat or plane surfaces directed away from the objective. The smaller of the two lenses is situated nearer to the eye of the observer and is known as the eyeglass; its function is to magnify the image formed by the objective. The larger, lower lens is known as the field or collecting glass; it renders the image clearer though, in so doing, it reduces the magnification of the eyeglass. In instruments provided with more than one eyepiece we shall wish to know which gives the greater magnification; this is or should be marked upon the metal rim surrounding the eyeglass but, in general, it may be stated that the shorter the eyepiece the greater its magnification. We repeat again, increase your magnification always, when possible, by using higher power objectives rather than eyepieces with greater magnifying powers. Sometimes it is necessary to use a greater magnification than our most powerful objective will give us; then we must fit our most powerful eyepiece and draw out the upper part of the microscope tube—in the best instruments they are made to pull out, after the manner of the telescope. The effect of so doing will be to increase the magnification considerably but, at the same time, the definition or clearness is seriously impaired.

For the examination of practically all our microscopic objects we require a number of slides, little glass slips of good, thin, clear glass. They may be used over and over again unless we make permanent preparations, but we are hardly likely to do so in our early days. The slides are held in place on the microscope stage, either by a pair of clips attached thereto or by resting against a bar running across the stage. We may here remark that it is essential always to keep one’s microscope slides absolutely clean. Dirty slides denote the careless worker; moreover, dirt when magnified is misleading. Objects which are being examined in water or any other liquid should be covered with a cover-slip, an exceedingly thin circle or square of glass. The cover-slip is as much a protection for the objective as for the object and its cleanliness also, is all important.

We have not mentioned any refinements such as the mechanical stage, by means of which slides on the stage may be rotated, moved to the front and to the back of the stage or from side to side. We have omitted these because they are not essential even for the very best work; they lend additional comfort to the use of the microscope but, again, they are not essential. The microscopist who requires such luxuries may learn about them in the larger text-books on the microscope.

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