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SUMMARY OF CURRENT RESEARCHES RELATING TO 



the interference figures may be made sharp. The lens may be focused 

 by a small rack-and-pinion attached to the body-tube. The prism-box 

 is adjustable, so that the analyzer may be removed from the optical axis 

 when not used. A slot, which may be closed, is provided for the 

 quartz-wedge, gypsum-plate, red of first order, or a |- undulation mica 

 plate. The coarse-adjustment is by rack-and-pinion. The micrometer 

 screw-head of the improved lever type of the fine-adjustment is divided 

 to give two speeds, and the larger part is graduated into one hundred 

 divisions, each one equal to ■ 005 mm. in vertical movement ; the 

 indicator is hinged. The polariser is a large Nicol prism in a revolving 

 mount graduated in single degrees and read by a fixed pointer. A stop 

 indicates the zero position, and an iris diaphragm is attached below the 

 prism. The polariser is used in the substage. A large Nicol prism, 

 mounted in a prism-box sliding in the body-tube, acts as the analyzer. 

 The illuminating apparatus consists of a two-lens condenser so as to 

 give converged polarised light. The upper lens is mounted in a metal 

 hemispherical shell, so attached to the mounting of the polariser that 

 it may be instantly thrown in or out of the optical axis without dis- 

 turbing any of the other parts ; it is operated by a conveniently placed 

 lever. 



Watson and Sons' Porro-prism Erector for Dissecting Microscopes.* 

 This apparatus (fig. (II) fits the firm's Laboratory and Simplex Dis- 

 secting Microscopes, and enables ordinary Micro- 

 scope objectives to be used instead of the usual 

 dissecting type. Greater working distance with 

 high powers is thereby afforded, and, as the image 

 is erect instead of inverted, dissection is facilitated. 



(3) Illuminating- and other Apparatus. 



Siedentopf's Dark-ground Illumination.t — 

 For dark-ground illumination with Siedentopf's 

 paraboloidal condenser, the Zeiss firm recommend 

 incandescent gas, spirit incandescent, the Nernst 

 light, or, best of all, the electric light, as sources 

 of light. With the aid of a spherical flask K (fig. 

 62), placed at a distance of about 15 cm. between 

 Fig. 61. the source of light B, and the plane mirror of the 



Microscope M, an image of the source of light 

 is projected on the latter, and at the same time effectually prevents in- 

 jurious heat affecting the specimen under examination. For instan- 

 taneous photomicrography of live bacteria, sunlight, projected on to 

 the mirror of the Microscope by means of a heliostat provided with a 

 clockwork motion, is in most cases essential. An exceedingly accurate 

 adjustment of all the component parts is also necessary. 



* W. Watson and Sons, Ltd., Catalogue, 1909, p. 69. 



t Carl Zeiss, Jena, Special English Catalogue (1907) Ultra-microscopy and 

 Dark-ground Illumination, part 4, p. 3. 



