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The Compound Microscope: Take

Two points are now needed; the first being to get the leaf flat, so as to avoid the undulation, and the second being to throw a proper light upon it. Take out the leaf, and, instead of laying it entire under the microscope, select the flattest part, and cut it out with scissors. A piece the size of a silver penny will be amply large enough. Lay this piece on the glass, get the focus afresh, and then look through the microscope. The leaf will now appear much more regular, and will be seen as a rough surface, mottled with white and traversed by pink and green ridges, which are the large and small nervures. By means of a mirror or the condenser throw a brighter light upon it, and it will be seen to be covered with a slight roughness, the nature of which cannot be clearly ascertained; then add the next highest power, and try if the structure of that roughness can be made out. Curiously enough, although the magnifying power has been more than doubled, the roughness has much the same appearance as before; so that we must try another plan, and look at the leaf edgeways. Take the piece of leaf in the stage-forceps, but do not touch it with your hand; fix the forceps on the stage and turn the leaf so that it presents its edge to the object glass. Get your focus, and you will now see the cut edge of the leaf, and will at once distinguish its structure. On either side may be seen the upper and lower cuticle, and in the centre the soft green substance, or "parenchyma," as it is called. From the cuticle project a number of short hairs, and when the focus is accurately obtained, the cause of the roughness will be seen in a vast number of minute projections, which are, in fact, identical in structure with the hairs, though not so well developed. The under-cuticle of the leaf is much more interesting than the upper. Now change the illumination, and, instead of throwing the light upon the object from above, turn the mirror so as to direct it through the object from below. No apparent result will follow, because the leaf is so thick and opaque that the light cannot pass through it. Hold the leaf horizontally, and, by means of the stage-forceps, rip it smartly across, and if you do this rightly, you will find that the two cuticles are partly separated, so as to allow either to be examined separately. At first the leaf will most probably be torn along one of the large nervures, so that the cuticles are not perfectly separated. Never mind failure, but try again; and you are sure, after a few efforts, to hit upon the right method of tearing the leaf. One of the most useful capabilities of the "live-box" is now shown. As may be seen by the figure and section, it consists of an inner tube with a thick glass, and an outer tube with a thin glass. The outer tube can be taken off, water or any other substance laid on the thick glass, and then the outer tube or cover is slid down upon it until the object is pressed flatly between the two glasses. When you have succeeded in getting a convenient slip of the leaf, lay it on the thick glass of the inner tube, and put a drop of water on it. Put on the cover, and push it down until the piece of leaf is pressed flat, without being squeezed. Now look through the microscope, and you will see a beautiful sight, showing how much there is in a despised leaf, which we daily tread under foot.

The cells of which the cuticle is chiefly composed are seen in many a waving outline, while at their points of junction are placed the remarkable contrivances called "stomata," or mouths, which are the apertures through which the atmosphere is enabled to penetrate into the interior of the leaf. The two semilunar cells at the sides of the opening may be considered as lips, which open and close according as the plant needs the air or not. The numerous dots which are seen upon the leaf are of a vivid green colour, and it is to their presence that the leaf owes its hue.

We have given these details because they are applicable to the examination of all leaves and petals, and show the young observer the method which is to be adopted when looking for the first time at a strange object. If the microscopist should follow up his work properly, and make sketches of every object which he places under the microscope, he cannot do better than use the camera-lucida, a neat little instrument, which is fitted into the eye-piece of the microscope. Dr. Beale's neutral glass is as efficacious in careful hands, and only costs a fourth of the sum. This instrument cannot be applied to the ten and twenty shilling microscopes, as it requires that the tube should be perfectly horizontal. The method of using it is simple enough. After fixing the object and getting the right focus, set the instrument horizontally, and arrange the light so that the object is well illuminated, and its lines quite clear and well defined. Now remove the cap of the eye-piece, and fix the camera-lucida in its stead. Lay a drawing-pad on the table under the camera-lucida, look through the square opening (or, if you use Mr. Beale's glass, look through the neutral glass), and you will see the object apparently projected on the paper. We say apparently, because in reality the image is not thrown on the paper at all, but on the camera, and the eye refers it to the paper, as being the nearest object. In fact, the principle on which this camera-lucida is arranged is exactly that of the Polytechnic ghost, which appears to be in one place, whereas it is in another. Now take a pencil, cut it to a very fine point, and trace the outline of the object on the paper. At first you will think this to be an impracticable task, for the point of the pencil will totally vanish. Soon, however, the eye will so adjust itself as to see the pencil and the object perfectly well, and by a little practice the observer will be able to sketch every object as rapidly and firmly as if he were copying a drawing, by means of tracing-paper. The neutral glass is perhaps to be preferred to the camera-lucida, as it is learned more easily, and gives less trouble than that instrument. Its cost is five shillings.