ZOOLOGY AND BOTA.NY, MICROSCOPY, ETC. 



683 



Fig. 137. 



can be turned round on this stem in such a way as to preserve this 

 angle constant for all azimuths. 



When it is placed so as to reflect light from the sky or a lamp 

 up the body of a Microscope, this reflected light is found to be plane- 

 polarized in the usual manner effected by 

 reflection from a bundle of glass plates. 

 It seems clear that the instrument is in- 

 tended for use as a polarizing mirror, and 

 its action is of the following kind. 



A .beam of ordinary light incident on 

 the prism at A emerges from the lower 

 face, when it falls on the latter at angles 

 less than the critical angle 41° 24', deviated 

 (and, of course,, also dispersed) to such an 

 extent as to fall on the bundle of glass 

 plates at the polarizing angle, 56°. It is 

 thus polarized by reflection in the usual 

 way, and passes upwards into the prism 

 near the edge B. In its passage through 

 the prism its dispersion is entirely cor- 

 rected, and it emerges as a colourless 

 plane-polarized beam in such a direction 



as to illuminate an object on the stage and enter the object-glass of 

 the Microscope. 



The main advantage which the apparatus was intended to secure 

 seems to be, to enable a ray to fall on the pile of plates at the 

 polarizing angle without the necessity of placing the plates very 

 obliquely to the axis of the Microscope. Thus there is a considerable 

 gain in convenience and compactness." 



Winkers Micrometer Eye-piece. — In such micrometer eye- 

 pieces as that of Gundlach (fig. 138), where the micrometer m is 



Fig. 138. 



Fig. 139. 



inserted in a slit (covered by a ring r) and the eye-lens o is focused 

 on the micrometer by moving it in or out, the magnifying power is 

 altered with each change in the position of the eye-lens. 



