We now have what is known as critical illumination, and the object to be viewed is brought into this brilliant image of the lamp flame after first removing the ground glass slide. As only a very small portion of the field is thus illuminated, this method is only suitable for testing resolving power and definition of the lens. The beginner had better, therefore, first obtain this critical illumination, and then rotate the lamp till its flame is broadside on to the mirror. The condenser should then be slightly racked down out of focus until the whole field is brilliantly illuminated. Now remove the eyepiece and look down the body tube and see how much light is entering the objective. To get good results, the back-lens should appear about two-thirds full of solid cone of light, and this must be obtained either by opening or closing the iris diaphragm, or placing a smaller or larger stop in the condenser. If possible, the eondenser should be fitted with an iris diaphragm, which consists of a series of thin metal plates so arranged that by moving a lever they can be made to open or close up to give any desired aperture.

We can now replace our eyepiece, and a good illumination for testing the general value of a lens will be obtained. The very finest definition is theoretically and practically obtained by easing the edge of the lamp-flame, viz., critical light, but for the beginner's purpose a fully illuminated field will be preferable. I might mention here that if the low-power objective is being used the condenser will enhance its performance. In this case, however, the top lens of condenser must be taken off, and, if possible, a circle of ground glass be placed in the stop carrier with whicb a condenser is usually furnished, so as to moderate the light. To return to our 1/4" or 1/6" lens, however, we shall now require a test object. The silicious valves of a species of organisms known as diatoms are generally employed for this purpose. These valves are mostly marked with a series of very fine, what will probably appear to the beginner as lines, but which under proper conditions assume more the appearance of short strings of beads or dots. The capability of the lens to show these markings as separate dots constitutes its resolving power, which is slightly dependent upon its aperture. In skilled hands, therefore, these tests objects are most useful indicators of the value of the lens being used ; but as a knowledge of the nature and physical characteristics of the object itself, and also the best methods of illuminating it to bring out these details, is essential to success, it would be better for the tiro to pick out an easier object.

If, however, he can obtain what is called a spread slide of mixed diatoms, he could use it to advantage in the following way. Having placed it on the microscope stage, he should first of all select a medium sized diatom under the low power, and bring it quite in the centre of the field. He should then substitute the higher power, focus it carefully, and see if the markings upon the diatom valve are shown in fine lines under a low-power eyepiece. If this is the case, the diatom will most likely be a suitable one for testing the lens, because on subtituting the higher power eyepiece, and manipulating the mirror so as to obtain oblique light, these lines may be split up into strings of pearls or beads. This result will depend entirely on the aperture of the lens, its perfection, and the skill of the observer in obtaining correct illumination. As the fineness of these markings differs considerably, even in the valves of the same kind of diatom, it follows that such a test must be a comparative one. Suppose, for instance, the observer can obtain the loan of a really good lens of similar aperture to his own, and can try comparison tests with his own lens on the same diatom, it will afford him a pretty good guide as to the capabilities of the one he has purchased. In passing I might mention that to obviate the variability in the fineness of the markings on diatom valves, test slides are often substituted artifically ruled with lines of such extreme closeness and exactness of individual distance apart that it requires very high magnification and big apertures to show them as separate lines.

These slides, however, are beyond the province of the beginner, but are interesting examples of human skill and ingenuity. To turn, however, to an object that can be judged with greater facility. Such a one would be found in the object used for testing the 1" objective, viz., the wing of the housefly. It must be viewed with critical illumination, using both low and high-power eye-pieces, noting carefully the appearance of the fine hairs fringing the edge of the wing. These should stand out sharp and clear, free from haze and color. Flatness of field, which, however, is not such an important property in a high-power lens as it is in one of the longer focal length, can be judged by racking down the condenser until the whole field is full of light, and then noting the sharpness of definition of the wing membrane at the sides of the field as compared with that in the centre. In using high powers an approximate focus should be obtained with the coarse adjustment, critical sharpness of image being obtained by using the fine adjustment without removing the eye from the eyepiece. Light rotation between finger and thumb of the milled head of the fine adjustment is all that should be nceessary to impart a smooth, low movement, with freedom from lateral displacement of the image.

A good general test object for our high-power lens, and one easily obtainable, is a drop of fresh blood. This can be easily obtained by tying a handkerchief tightly round the forefinger, which should first of all be held downwards for a short time, and then pricking the tip of the finger with a perfectly clean fine needle. In pressing the finger, a minute drop of bood will exude, and this is transferred to a slide as follows: - A thin clean 3" x 1" slide is placed on a piece of notepaper, and a thin cover-glass is put on its centre. The blood-drop is then touched against the edge of the cover-glass, when capillary attraction sets in, and a thin film of the liquid is drawn under, and is then ready for examination. When viewed with a 1/4" or 1/6" lens, human blood is seen to consist of numerous rounded biconcave corpuscles floating about in a colorless liquid. On closer examination a few larger colorless or greyish corpuscles will be seen, these being the so-called white, the others the so-called red corpuscles of the blood. The sharpness of outline of the margins of the red corpnscles should be carefully noted, and a general view of the field taken before arriving at any conclusions as to the value of the lens.