ZOOLOGY A>rD B0TA2TT, illCKOSCOPT, ETC. 



573 



the position of exact correspondence of the outer and inner threads. 

 This is apart from the objection common to all these forms of adapter, 

 that they are liable to let the objective drop off when the adjustment- 

 collar is turned, especially if it moves somewhat tightly. 



Zeiss's Stage-Micrometer. — This micrometer (fig. 106) is intended 

 more particularly for the measurement of objects, the whole of which, 

 cannot be seen in one field of view. 



The upper plate is graduated, and rotates on the middle plate. 



The latter is moved laterally on the lower fixed plate by the screw 

 shown on the right. The number of whole turns of the screw is regis- 

 tered on the small dial with index, by means of an endless thread on 

 the periphery of the drum, working on a toothed wheel on the same 

 axis as the index, while parts of turns are shown by the graduations 

 on the drum of the screw. 



Queen's Holder for Woodward's Prism. — Fig. 107 shows an 

 arrangement issued by J. W. Queen and Co., of Philadelphia, for readily 

 applying the Woodward prism. The prism is mounted between jaws 



