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other in the eye-piece it may be then noted how many divisions of the micrometer are equal to a certain number on the stage micrometer. If, for example, 2 divisions of the stage micrometer ( = 20 ) correspond with 7 divisions of the micrometer of the eye-piece, the apparent size expressed in divisions of the eye-piece micrometer have to be multiplied by 0 = 2:857 in order to get the true length expressed in . The higher the magnification, the smalleris this factor.

Counting Apparatus. In order to be able to count yeast cells a “net "eye-piece is used, consisting of a circular piece of glass on which is etched a square, divided up

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a (a) The object glass ; (6) the cut-out cover glass fastened on to it. (After Hayem-Nachet.)

into sixteen or twenty-five smaller squares; it is fitted up in the same way as the micrometer in the eye-piece and is used in conjunction with a hæmatimeter. This (Fig. 7) is a glass slip (a), to which is fastened a cover glass (b), from the middle of which a circular piece has been cut out. A drop of liquid containing the cells to be counted is placed in the shallow space thus formed and enclosed by an ordinary cover glass. The thickness of the perforated cover glass is usually 0:1 or 0.2 mm.

Thoma's hæmatimeter (Fig. 8) is also used for counting micro-organisms. A is a glass slip, on which a cover glass (a) is fastened which has a circular hole in the middle and is 0.2 mm. thick. A circular cover glass (c), 0:1 mm. thick,

is fitted centrally in this hole and is also fastened to the glass slip; thus, an annular space (d) is formed. In the middle of (c), two sets of twenty-one parallel lines are etched which cut each other at right angles. There are thus formed a large square with a side of 1 mm., and small squares with a side of 0.05 mm. The drop of liquid to be examined is placed on this square and enclosed by the cover glass (6), the depth of the liquid layer (e) thus formed amounting to 0.1 mm. B gives a vertical section of the chamber.

The method of counting is described later.

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Fig. 8.-Thoma's Chamber. (A) View from above ; (B) from the side ; (6) a

cover glass. (The significance of the remaining letters is given in the text.)

Klönne and Müller's Object Marker, — The object marker (Fig. 9) made by Klönne and Müller (Berlin) is often used in the course of the preparation of pure cultures and during observations on growth.

This instrument is used to mark a certain spot in a preparation by impressing a coloured ring round it on the cover glass. The apparatus is fitted on the microscope in place of the objective. There is an opening where the front lens of the objective would otherwise be. The lower half of the apparatus is capable of vertical movement, being fitted with

a somewhat weak spring. The opening at the point must be quite flat and ought not to have a greater diameter than 0.75 mm. The method of using it will be referred to later.

Cover-Glass Gauge.-In the above it has been mentioned that objectives are sometimes provided with a correction so as to be adjustable for any thickness of cover glass. It is occasionally important to be able to measure the thickness of cover glasses. The instrument represented in Fig. 10 is used for this purpose. When the upper part

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is screwed down as far as possible, the zero of the scale on the screw-head coincides with a mark on the stem. In measuring the thickness of a cover glass the screw is screwed back, the cover glass inserted edgeways in the space thus made, and the screw again brought down until it touches the cover glass. The number which is now at the mark gives the thickness in ito mm.

Apparatus for Artificial Illumination. In microscopi. cal work one must often resort to artificial light and so lamp is necessary on the microscope table. Electric light has the advantage of coolness; otherwise a gas or oil lamp

must be used. As the continued use of a strong, yellow light is injurious to the eyes, it is better to let the rays from the lamp pass through a blue liquid, by means of which an agreeable greenish or bluish light is obtained which does not affect the eyes. Such a liquid may be prepared by adding ammonia to an aqueous solution of copper sulphate contained in a glass globe. The best illumination is then obtained by a suitable adjustment of mirror, globe and lamp. The positions of the microscope, globe and lamp can then be marked on the table so that these can easily be put into place at any time.

In addition it is as well to fix a screen between the microscope and the lamp, partly to protect the eyes from the direct light of the lamp, and partly on account of the heat.

If a gas lamp is used as the source of light, a damp cloth should be hung on the screen to keep the air moist.

Small Auxiliary Apparatus. In addition to what has been described, various small articles are required, e.g., glass spatula, platinum wire, forceps, preparation needles, etc. Two glasses may also be kept on the table for holding dilute sulphuric acid; in one, soiled glass slips are put, in the other, cover glasses; in this way the micro-organisms on the glass are killed and thus prevented from passing into the air after the glass has dried and before it is cleaned again.

Bottles for Reagents and Immersion Oil.—For microscopical work certain chemical reagents are indispensable. These are preferably kept in bottles provided with glass stoppers which are drawn out into the form of a rod; the latter reaches almost to the bottom of the bottle and in some cases has a thickened end. Figs. 11 and 12 represent two of the commonest forms of these bottles.

Fig. 13 represents a bottle which is made by Leybold's successors (Cologne) according to the design of Arthur Meyer, and which has found general acceptance. It has this advantage, that the rim, which is enclosed in a cap, cannot be easily soiled since this is bent inwards into a funnel shape which compels the operator to draw the glass rod out of the middle of the bottle, the rim automatically removing superfluous liquid. The liquid which is removed at the funnel runs back again into the bottle; the latter should be kept quarter full, as, if upset, the rim prevents the liquid from running into the cap.

Fig. 11.-Bottle for micro- Fig. 12.---Bottle for micro- FIG, 13.-Arthur Meyer's

chemical reagents or im- chemical reagents or im- bottle for micro-chemi. mersion liquid. mersion liquid.

cal reagents or immersion liquid.

4.Thermostats and their Accessories. Certain constant temperatures are necessary in fermentation work; such apparatus as will accomplish this and in which cultures can be placed are called thermostats or incubators. There are now numerous designs of these; we shall only mention a few here which have been found suitable. One of these, for instance, is the thermostat made by Rohrbeck (Berlin) which is depicted in Fig. 14.

Rohrbeck's Thermostat.-This consists of a doublewalled copper box oval in section, covered with felt on top

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