158 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 



ghenian and other eyepieces in this respect, that 

 the eyelenses of even the strongest have relatively 

 long foci,, so that they can be used with almost as 

 much comfort as the lower power oculars. The 

 camera lucida can also be used with any of them 

 save the highest, which magnifies 27 diam., and has 

 10 mm. focal length. These are great recommenda- 

 tions, and the wonder is that opticians have not 

 long since constructed high magnifying eyepieces 

 with large eyelenses of such focal lengths that 

 comfort is insured thereby and undue straining of 

 the eyes avoided. 



Instead of naming the different eyepieces ABC, 

 &c, or 12 3, &c, as other opticians do, Dr. Zeiss 

 designates the compensating oculars by their magni- 

 fying power; thus the eyepieces No. 1, 2, 1, 8, 12, 18, 

 27 magnify the image produced by the objective 1, 

 2, 4, 8, 12, 18, 27 times respectively. Both the magni- 

 fying power or number and the focal length are 

 engraved on each eyepiece, so that when the magni- 

 fying power of an objective at the end of a tube 

 160 mm. or 250 mm. long is known, one can at once 

 find the magnifying power of the microscope by 

 multiplying the initial magnification of the objective 

 by the number of the ocular with which it is 

 combined. 



The compensating eyepieces are divided into three 

 classes — viz. : 



(1) Search oculars of great focal length. The one 

 No. 1 constructed for the short or Continental tube 

 does not magnify the initial magnification of the 

 objective at all, and the two No. 2 magnify, both 

 on the Continental and on the English tubes re- 

 spectively, the image produced by the objective two 

 diam. These objectives, as their name indicates, are 

 intended to enable the observer to find rapidly an 

 object in the field without employing another low- 

 power objective, and perhaps a cumbersome nose- 

 piece, which is so prejudicial to the centricity of the 

 optical system and to the fine adjustment of the 



