duced by Amici, whilst homogeneous or oil immersion was first suggested by Stephenson.

In order to understand the advantage of the immersion system over the dry it is necessary to know what the angular aperture and the numerical aperture of the lens are. The angular aperture is the greatest angle formed by two lines drawn from the focus to the edge of the lens. The numerical aperture is the product of the refractive

FIG. 3.-Diagram of a section through the front lens of the objective and the cover glass, showing the direction of the different rays of light with and without immersion liquid.

The

index of the medium between cover glass and objective and the sine of half the angular aperture (Fig. 3). greater the numerical aperture the more rays pass from the object through the objective. In the dry system the numerical aperture is always less than 1, for the refractive index of air is equal to 1, and half the angle of aperture must, of course, be always smaller than 90°, and its sine

consequently less than 1. The refractive index of water is 1-33, but in practice the aperture for water immersion is never greater than 125; for cedar-wood oil and glass the index is the same, viz., 152, but the aperture is not more than 1.40. The difference between the immersion and dry systems may be understood from the accompanying sketch (Fig. 3). This gives a section through the front lens, L, of the objective, the cover glass, and the intervening medium, that to the right through air (refractive index м 1), that to the left through oil (μ 15). Two rays, EO and GO, proceeding from the object are refracted in passing through the cover glass towards the normal OP; GO in the direction D, EO in the direction J. When the ray, GOD, leaves the cover glass it passes into the immersion liquid, in this case oil, which has the same refractive index as the cover glass; the ray therefore will continue through the oil in the same direction as through the glass and will pass into the objective at A. If, on the other hand, the dry system is used instead of the immersion there will be air between the cover glass and objective with refractive index μ= 1, so that a ray, EO, which leaves the cover glass at J does not pass into the objective at all. Only those rays which are less oblique than some ray such as FO will be able to strike the objective. It is thus seen that far more rays take part in the production of an image when the immersion system is used, and in consequence of this a much better image is obtained.

The strength of the oil immersion lenses is usually indicated by a fraction, e.g., 1, 1, etc. By this is meant the equivalent focal distance of the respective lenses, expressed in inches.

Immersion lenses ought to be cleaned immediately after use by means of an absolutely dust-free linen rag which should be kept in a tightly closed box. A quite dust-free

material should also be used for drying the lenses, for dust often contains particles which are capable of scratching glass. However, the lenses themselves should be moved and rubbed as little as possible. The liquid can be removed from the edge of the glass, when necessary, by means of blotting paper, and under certain circumstances the glass may also be cleaned with benzene or alcohol. The other parts of the microscope can be dusted by means of a soft brush.

The Stage is fitted up in various ways, e.g., so that it is capable of rotation and of adjustment in different positions. These are, however, details, the description of which may be omitted.

There is sometimes a scale and vernier on the stage, the vernier being an arrangement which allows a finer reading of the scale to be made. It is a smaller scale running parallel with the fixed scale, and ten of its divisions are exactly equal to nine divisions of the latter. That division of the vernier which coincides with a division of the stage scale gives the required fraction in tenths.

Changing the Objectives. There are various ways of attaching the objective to the tube. The objectives, which in some cases are simply screwed into the tube, may also be adjusted by means of a revolving arrangement (nosepiece), allowing several objectives to be attached to the tube at the same time; by simple rotation any objective may be brought into the required position. The objectives can also be changed by a sliding arrangement or with the aid of clips. These different arrangements enable one to change the objectives more easily and quickly.

Some general rules for the testing of a microscope will be given in what follows.

Testing the Microscope. There are usually given with each microscope several test objects; most frequently these

consist of scales from the wings of a butterfly (Epinephele janira) and a diatom (Pleurosigma angulatum). The microscope may then be tested by examining these under different magnifications. The latter are to be found in the table always supplied with the microscope. The first preparation is examined with a low magnifying power (60 to 150 times); the transverse marks on the scales ought then to be quite distinct. The silicious shell of the diatom has fine crossed markings; with a magnification of 400 to 500 these ought to be plainly visible; with a magnification of 150 to 200 and using oblique light they ought to be distinguishable. To obtain oblique light the concave mirror is turned so that the light enters from one side. With very strong immersion lenses the markings are resolved into a mass of small six-sided figures. In such a test the sharpness of the outlines ought also to be noted. Lastly, it should be ascertained whether the screws, etc., fit well.

When the microscope is not in use, it must be covered up so as to protect it from dust. A bell jar is to be recommended for this purpose, that half of it turned towards the light being painted with oil colour to protect the microscope at the same time from sunlight which, in course of time, affects the fine lenses, the mirror and the stand.

Slips and Cover Glasses. In the microscopical investigation of micro-organisms glass slips and cover glasses are used. The growth is placed on the slip in a liquid and the cover glass laid on the top. The slips are rectangular, and in general 75 cm. long, 2:5 cm. broad, and 15 mm. thick. Cover glasses are of various thicknesses, thin ones being used for finer work. The use of thick glasses, e.g., 0.20 mm. thick, is to be recommended for ordinary work. If mixed cover glasses are bought, they ought to be sorted according to their thickness, which can be determined by means of the apparatus described on page 34. The cover glasses

used most are 18 mm. square; but round and rectangular ones of various sizes are also employed.

For several kinds of work cover glasses should be used which are etched with squares, the squares in some cases being numbered. Fig. 4 represents a squared cover glass, used by Hansen in his first pure culture method. The

FIG. 4. Hansen's Squared Cover Glass.

squares were used for determining the total number of cells. which were present in a drop of any liquid. This drop was contained within the boundaries of the whole square. Hence the small size of the latter.

The most frequently employed squared cover glasses have larger squares (see Fig. 5 and Fig. 6). Will only numbers the squares in the top row and the left-hand

column, but Alfr. Jörgensen numbers all the squares (Fig. 6).

Since such cover glasses are somewhat expensive and can be easily etched, we describe a method given by Will for this purpose. A little wax is melted in a saucer and the cover glass dipped into it, being held at one corner by