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with dilute spirit. The dimensions are about as follows: height, 56 centimetres (22 in.); length, 63 centimetres (25 in.); breadth, 50 centimetres (20 in.). The front side consists of a sliding door which can be kept open at any desired height. Before the cupboard is used it is washed inside and out, either with boiled water alone, with per cent. solution of mercuric chloride, or with dilute spirit; it is specially important first to brush and then to damp the surface where the door slides up and down, in order to prevent germs which settle in the groove from penetrating into the cupboard.
The cupboard is then closed and allowed to remain (usually for an hour), till the air inside has become quite still, and the water particles with which it is saturated have carried the germs present down tc the damp floor.
The cupboard must be kept sufficiently damp during experiments, as otherwise the germs are apt to be stirred up again.
In experiments where infected solutions or the like are liable to be spilt, it is an advantage to cover the floor of the cupboard with a zinc tray which can be easily removed and which can be cleaned and sterilised before and after use.
3.-The Microscope and its Accessories. The microscope is one of the most important adjuncts in investigations connected with the physiology of fermentation. It will be shortly described here, and, in addition, references will be made to special literature where more detailed information may be obtained.
The compound microscope (Fig. 2) consists of two systems of glass lenses, the one nearer the object of investigation being called the objective, and the other nearer the eye, the eye-piece. All these lenses are fitted into a brass tube. The objective forms a real, enlarged and
inverted image of the object which is being examined ; this image is again magnified by the eye-piece. Thus the image that we see in a microscope is an inverted one. However good the lenses may be they never form exact images, and the error is increased by the eye-piece.
Spherical and Chromatic Aberration. — The errors caused by the objective arise mainly from spherical and chromatic aberration. Spherical aberration is to be ascribed to the fact that, of the rays of light which, diverging from
a point, pass through a lens, the central ones are not focussed at the same point as the outer ones. The image is therefore hazy in outline. Further, white light, as is known, is composed of different coloured constituents, and on passing through the lens these are separated so that a coloured image is produced and the outline contains the familiar rainbow colours. To reduce spherical aberration various diaphragms are inserted in the tube which cut off the peripheral rays, and to eliminate chromatic aberration the lenses are composed of a biconvex and a plano-concave lens made respectively of crown and flint glass. This method gets rid of chromatic aberration almost entirely. In order to further correct for this and spherical aberration there is placed between the objective and the eye-piece a so-called collective lens.
Achromatic and Apochromatic Objectives. - Such objectives as we have described are styled achromatic; recently so-called a pochromatic objectives have also been constructed. These are made of special kinds of glass (borate, phosphate, baryta and fluoride glass), by means of which more perfect colour correction is attained. They are far more expensive than the first named, which are quite good enough for the ordinary demands of fermentation work.
The Tube,—The tube is so arranged that it can be elongated and can thus increase the magnification; frequently it is provided with a scale of divisions by which the lengthening can be determined. The tube is supported on a brass stand which carries, among other things, two screws, one for coarse and one for fine adjustment.
Correction Objectives. - Objectives for high magnifications have in some cases an additional adjustment for the varying thickness of cover glasses. In these there is a ring on the objective provided with a scale, the numbers of
which correspond with cover glass thicknesses expressed in tenths of a millimetre; the ring is turned until the proper mark coincides with a fixed index.
The Condenser. — A centrally perforated stage, on which the preparation to be examined is laid, is also fixed to the stand; under this stage there is a mirror which serves to project the rays of light through the aperture in the stage, through the preparation, and so along the tube to the eye, thus providing the necessary light for observation. The mirror is double, being plane on the one side and concave on the other; the concave side gives the strongest light and is therefore used for the higher magnifications. Of late years other condensers have been used, especially that designed by Abbe. This apparatus, which may be seen in Fig. 2, lying in front of the microscope, causes a much more intense light to pass through the microscope. The light is regulated by means of a diaphragm, for it may be so intense as to make the preparation indistinguishable. With increasing magnification more light is required. A very suitable form of diaphragm is that known as the iris diaphragm (brought forward in Fig. 2), which can be easily adjusted so as to allow more or less light to pass through. If the Abbe condenser is not used there are circular diaphragms with openings of different sizes which can be brought under the aperture of the stage. The condenser is especially advantageous in the investigation of stained preparations of bacteria.
Immersion Objectives.-For very high magnifications, and in order to get specially good definition, immersion objectives are used. In these the objective lenses are very powerful (and therefore very small since their curvature is great), and are immersed in a drop of liquid (water or oil) which lies on the cover glass. Water inmersion was intro
duced by Amici, whilst homogeneous or oil innmersion 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.
index of the medium between cover glass and objective and the sine of half the angular aperture (Fig. 3). The 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