15. After removing surplus alcohol, pour on a few

drops of essential oil (origanum, xylol).

16. Remove oil and mount in xylol-balsam.

Dehydration and mounting.

N.B.—In making a balsam preparation, the sections must always pass through four groups of fluids.

Watery liquid-Alcohol-Essential oil-Balsam.

→ 3

→ 4

Never take an object from one group to another of the series without passing it through the intermediate group; it must be passed from the 1st to the 4th, and on the return from the 4th to the Ist.

PART II.

LESSON I.

MILK, GRANULES, FIBRES, AND VEGETABLE ORGANISMS.

1. Examine the Microscope, the objectives, and the eye-pieces. (a.) Select the objective and ocular required. For a high power (H), if a Zeiss' microscope be used, select the objective D and the ocular 2; if Hartnack's, the objective No. 7 and the eye-piece III. See that the lenses are clean. Place the ocular in the tube, and screw the H lens to the lower end of the tube, and leave it half an inch above the level of the stage. For a low power (L) use No. 2 ocular of Zeiss or III. of Hartnack, and objective A or No. 3 respectively. In using a low power, the lens must be 1 inches above the stage to begin with.

(b.) With the microscope in front of you, with high-power lens on it, arrange the concave side of the mirror under the stage so as to reflect a beam of light up the tube of the microscope into the eye, looking in at the ocular. Turn the sub-stage diaphragm until a small aperture in it is under the aperture in the centre of the stage. If any specks are visible on looking through the microscope, rotate the ocular; if they move, of course they are on the ocular itself. Clean the outer surfaces of the lenses of the ocular with a piece of clean wash-leather, which should be kept tied to the microscope and used for no other purpose than cleaning the lenses. Replace the ocular, and if specks are still present and move when the ocular is moved, they must be on the inner surface of the eye-glass or fieldglass of the latter. This is easily determined by rotating the eyeglass of the ocular alone, while looking through the microscope, and observing if the specks do or do not move with it. Clean the inner surfaces of these lenses. A general dimness indicates that the objective itself is dirty. The light used should not be direct sunlight, but preferably light reflected from a white cloud.

The rule with regard to the use of the diaphragm must never be neglected, viz., to use a small aperture with a high power, and a large aperture with a low power.

2. Clean a Slide and Cover-Glass.

(a.) The Slide.-Seize the slide by its edges with the thumb and forefinger of the left hand, dip one half of it into water, withdraw it, and with a clean old handkerchief rub both wetted surfaces at once until they are clean and dry. Reverse the slide, still holding it by its edges, and dip the other end in water, and clean its surfaces as before. Lay the slide upon some clean, suitable background, white or black paper, or on the photophore.

(b.) The Cover-Glass.-Sometimes the covers have a thin film on them; this may be got rid of by placing them in strong sulphuric acid, and subsequently removing every trace of acid by water. Dip the cover-glass in water, take it between two folds of a handkerchief held between the thumb and forefinger of the right hand, and rub both surfaces at once. After it is cleaned, do not lay it flat, but tilt it up against some convenient object.

The first lesson is devoted to the examination of a few simple objects some of which are occasionally found as foreign bodies in microscopical preparations-with a view to familiarise the student with the use of the microscope.

3. Milk. By means of a glass rod, place on the centre of the slide a small drop of milk diluted with three or four volumes of water. To find the centre of the slide, use the mounting block (p. 89).

Apply a Cover-Glass.-Seize the cover-glass by the edge by means of a pair of forceps with broad points. The pattern shown in fig. 6 is convenient. The edge of the cover-glass opposite to the forceps is allowed to touch the slide close to the drop of fluid, the edge opposite being gradually and evenly lowered by depressing the forceps until the fluid touches the under surface of the cover-glass. By lowering the cover-glass thus gently and obliquely the entrance of air-bubbles is avoided. Place the object on the stage right under the lens.

Focus the Object (H).—The objective is still half an inch above the stage. While looking into the eye-piece of the microscope, seize the tube of the latter between the thumb and adjoining fingers of the right hand, and with a screwing or twisting movement of the tube from left to right, gradually depress the tube until the outlines of the object are indistinctly seen. This is the coarse adjustment. The focussing process is facilitated by keeping the slide and object moving slightly. This can readily be done by moving the slide with the thumb and forefinger of the left hand, the ulnar margin of the palm conveniently resting on the table.

Now use the fine adjustment, and bring the outlines of the object in the field sharply into view.

It is of the greatest importance that the student should be taught to describe the objects which he sees, and also to make sketches of them. To facilitate the description of isolated objects, the following heads may be adopted :

In the object under examination there is a large number of minute bodies floating in a fluid. Describe the appearance of the floating particles under the following heads:

(a.) Shape. The milk globules (fig. 52) are spherical, as can be shown by touching the edge of the cover-glass with a needle, and then observing them as they

rotate in the field of the microscope. Moreover, if one be focussed, its outline comes gradually into focus, and disappears gradually, while optically with regard to light these bodies behave as globules, and not as discs.

throughout its entire thickness, until its under surface is brought into view. Both surfaces are smooth.

(d.) Size. The globules are not all of the same size. If desired, measure their actual size (p. 20).

(e.) Colour.-The smaller ones appear colourless, but some of the larger may have the slightest tinge of a faint yellow.

(f) Transparency and relation to light.-They are transparent, because the outline of a subjacent one can be seen through a globule

lying over it. Notice also the highly refractile character of each globule, characteristic of an oil droplet (fig. 55, 3).

(g.) Contents.-They appear homogeneous and uniform, and no included body is to be seen. Each globule is, in fact, a globule

of oil.

(h.) Effects of reagents.-To one side of the cover-glass apply a drop of acetic acid. To the opposite edge of the cover-glass apply the apex of a triangular piece of blotting-paper. The blottingpaper sucks up some of the milk, and the acid runs in at the opposite side to supply its place. This is the process of irrigation. Move the slide to bring into focus a part of the field which has been acted on by the acid, and note that the corpuscles, instead of floating about singly as before, are now aggregated into small groups. The acid seems to have altered the surfaces of the globules, so that they adhere to each other. The acid is said to act on the casein envelopes of the globules, and to soften or dissolve them. This preparation is not to be preserved.

Make sketches of these objects before and after the action of reagents.

4. Potato-Starch Granules. With the blade of a knife gently scrape the surface of a freshly-cut raw potato; place the matter so obtained in a drop of water on a slide. Remove any coarse fragments, and apply a cover-glass. Focus the object (H).

(a.) Observe that the granules (fig. 53) are ovoid bodies of unequal size, not equal at the two ends, clear, and with a sharp

Potato-Starch.

outline. Near the smaller end of each granule notice a small spot, the "nucleus" or hilum, round which are concentric layers, giving rise to the appearance of fine concentric lines arranged with relation to the nucleus. The lines are more numerous on one side of the hilum than the other. Sketch two or three of the granules.

(b.) Irrigate the corpuscles with a diluted solution of iodine in iodide of potassium. Each

FIG. 53. Granules of granule becomes blue as the iodine reaches it. This is due to the formation of iodide of starch. If the iodine be too strong the granules appear black.

5. Rice-Starch (H).-Examine a little rice-starch in the same way. Notice the much smaller irregular granules. Each granule is polygonal, mostly five or six sided. The granules are stained blue by iodine (fig. 54). Make sketches of the starch corpuscles.

6. Gamboge and Brownian Movement (H).-Rub up a small piece of solid gamboge in water until the latter has a faint yellow appearance. Place a drop on a slide, cover, and examine.

a.) Observe granules of various sizes and shapes floating in the