S. E. Dowdy. M. P. S

It now only remains to describe the objectives and eyepieces. Microscope lenses, objectives, or powers as they are variously termed, consist of combinations of lenses of varying magnifying power, mounted in short brass tubes furnished with a standard size screw for attaching to the lower end of the microscope body-tube. The gauge recommended by the Royal Microscopical Society is the one almost universally adopted by makers nowadays, and in no case should an instrument be obtained having any other gauge for its lenses.

There is little fear of getting such an instrument new; but some of the older makers might differ in this respect. To distinguish them from one another and indirectly to indicate their magnifying power, English lenses are designated in inches and parts of an inch, Continental ones by millimetres. For instance; an English lens might be a 1/2", 1/4", and so on, down to 1/40" oar1/50 of an inch, whereas a lens by a German maker might be a 24, 12, or 6 millimetre, as the case might be.

The beginner is naturally apt to fall into the error that, because a lens is called, say a 1", that it will -focus at exactly that distance from an object. This is not necessarily the case, the terms applying to the focus of a single lens of the same magnifying power. The objective may be an optical combination of half a dozen lenses, the front one of which may and probably does focus much nearer to the object than the objective's designation would imply.

The value and cost of an objective is partly dependent upon its magnifying power, but more so upon what is termed Its numerical or angular aperture. It is difficult to explain the meaning of this term in a word or two, but briefly it refers to the angle at which the rays of light from an object can enter the objective to form a well corrected image. This is not dependent on the size of the front lens of the objective, as a beginner might expect, but is a question of the curvatnre of the various lenses making up the combination. A lens of wide aperture, providing the proper corrections have been made in it, will admit more light and show more detail than a narrow apertured lens of the same magnifying power. It is therefore superior to the latter, but owing to increased skill and work required for its manufacure, is much more expensive. Students' lenses are now made with larger numerical apertures than they used to be, owing to a more general recognition of the fact that on the aperture depends the resolving power and definition of the lens, providing workmanship is good. The bearing that aperture has on the performance of a lens will be noted, when we come to the testing of lenses, as it is rather an important point. We now come to the eyepieces.

Those that the student will require are what are known as Huyghenian, consisting of a couple of plano-convex lenses. mounted in a short metal tube. The lower lens is termed the field lens, the upper the eye lens. Eyepieces are constructed of various magnifying powers from about 4 diameters up to 18 or 20. Low power eyepieces, which are the most useful, are sometimes called shallow, those of higher power, deep eyepieces. A student's stand is generally supplied with one low-power eye piece, magnifying four or five diameters, but an extra one of rather high power is well worth obtaining, as they can be obtained for $5 each.

It is of little use for the beginner to buy an eye piece of higher power than one magnifying about ten diameters, for the following reason : - In observing an object with a compound microscope, the image formed by the objective is further magnified by the eyepiece. Any imperfections, therefore, in this primary image are accentuated by the eyepiece in proportion to its magnifying power. Carefully corrected lenses of wide aperture will stand deep eyepiecing without appreciable deterioration of the image ; but students lenses, though giving excellent results with low power eyepieces, break down under high power ones. This is the reason why, in testing micro-scope lenses, the appearance of the image is noted under a deep eyepiece.

We have now briefly enumerated the leading features of a student's microscope, and the novice is advised to make himself acquainted with the specific purpose of each part of his instrument before proceeding to do work with it. Each observer has different methods of working, but it is he who has the better knowledge of the capabilities of his microscope who will make the best microscopist.

The next thing after, or better still, the first thing before purchasing a microscope is to test its objectives, eyepieces, and adjustments. Take nothing for granted, but carefully see that each adjustment is efficient, and above all things, that the lenses are of good quality. For actual working, daylight, for several reasons is preferable; but for testing lenses, artificial illumination admits of more control, and will serve our purpose better. For providing the necesaary light a small kerosene lamp will be required, any hand lamp with a 1/2" wick will do. The microscope should be placed on a steady table or bench, with the lamp on the left-hand side, and slightly in advance of the instrument. The probability is, if the microscope has been purchased complete, that it will possess a couple of objectives, a 1/ or 2/3" and a higher power of from 50 to 500 diameter may be obtained.

We will now start with the low-power lens, as it is easier to use. Screw it into the lower end of the body-tube, and place the low-power eyepiece in the upper end of the tube. Now manipulate the mirror so that a beam of light is thrown up through the stage aperture into the field of the lens. On looking through the eyepiece, a brilliant circle of light should be seen, this being called the field. If unevenly illuminated, a fresh adjustment of the mirror, or a slight alteration in the position of the lamp will be necessary. If the microscope is fitted with an Abbe substage condenser, the top lens of condmser should be first removed before adjusting the light. It is, however, with the higher powers that the sub-stage condenser comes in useful, so that a few words will be said with regard to the method of using it when we are testing the high power lens. Now remove the eyepiece and look down the tube of the microscope, and see how much light is entering the objective. If full of light, rotate the diaphragm under the stage until the cone of light from the mirror is cut down until it illuminates about two-thirds of the back lens of the objective; then replace the eyepiece. We shall now require a test object, which is one possessing fine enough markings or detail to require a good lens to show them well. Most opticians supply suitable objects already mounted, such as the tongue of the blow-fly, for low powers, and diatoms and insect scales for high power lenses. As recourse to an optician is not always available, we will select an object readily obtainable, at least, during a good part of the year, easily mounted, and furnishing a good test of the capabilities of a lens. I am referring to the wing of the common house fly. After killing the insect, detach one of the wings and place it on a thin 3" x 1" glass slide, cover it with a thin square cover glass, which should be held down by a couple of strips of postage stamp paper.

The object is now ready for examination, the slide can therefore be placed on the microscope stage, with the object in the centre of stage apature. Now use the coarse adjustment by either sliding the body tube up or down or lightly rotating the milled heads provided for the purpose. When the image is coming to a focus, note the working of the coarse adjustment. A good rack-and-pinion adjustment should give a smooth, gliding motion free from jerks, and on looking at the object no displacement to one side should be noticed on altering the focus. When a sharp focus has been obtained, carefully note the appearance of the object, which will present the appearance of a delicate membrane studded with minute hairs and traversed with thickenings of the membrane, which thus form a framework. A fringe of very fine hairs will be found edging the wing, and particular attention should be directed to these. With a good lens each hair will stand out perfectly sharp and clear, with no trace of a colored fringe surrounding it, and the hairs on one side of the field of view will be in as sharp focus as those on the opposite side. Now pull out the draw tube, if the microscope is fitted with one, and re-focus. Increased magnification is the result, but the defination will probably fall off a bit, partly due to the lens probably being corrected for a short tube length. Push in the draw tube and substitute the high power eyepiece, and note the effect. A less brilliant image will be the first noticeable alteration, also, of course, an enlargement of the image. No further detail will be shown, but we have now to note what appearance this enlarged detail presents. If the hairs on the edge of the wing still present a perfectly clear, sharp cut outline, and are free from color, the lens may be relied on for any work the student is likely to require it for.