EYEPIECES 73 



use with apochromatic and high-power achromatic or 

 fluorite lenses, and are only made to give small magni- 

 fications, 2 and 4 times for the 160 mm. tube, which 

 become 3 and 6 times if used on the 250 mm. 

 tube. The 4 is the more useful eyepiece of the two. 

 When in use an image of the diaphragm they carry 

 must be focussed on the ground glass by rotating the 

 movable eye-lens (Fig. 23). The graduations serve to 



Fig. 23 



PROJECTION EYEPIECE 



record the position necessary to obtain accurate cor- 

 rection for any camera extension. In practice it is 

 somewhat difficult to get the best focus of the diaphragm, 

 and Messrs. Zeiss supply a tape by which the extension 

 of the camera can be measured, and which indicates 

 by a number corresponding to the graduations the best 

 position of the adjusting ring for that extension. 



Demonstration Eyepieces are made by Leitz with 

 a second eye-lens, through which part of the light is 

 deflected by prisms, in order that two observers may 

 examine a preparation at the same time, and they are 

 very useful for instantaneous or other photomicrography 

 in which it is necessary to watch and focus the pre- 



