INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 



1053 



in England, but instead of the movement of the plates being 

 controlled by a lever arm, there is an outer cylinder-cap A, that 

 can be turned like the adjusting collar of an objective. The range 

 of aperture is from about ^ inch to a pin-hole, and it remains 



Fig. 122. 



in the same piano during the motion. This diaphragm is mounted 

 on a substage B provided with centering motions (a short bar 

 working with a loosely fitting slot, that can be clamped beneath), 

 which is a somewhat primitive contrivance. Centering motions must 

 be capable of exact adjustment or they are practically useless. 



Swift's Calotte Diaphragms. — Messrs. Swift have recently devised 

 two forms of " calotte" diaphragms for use above the achromatic con- 

 denser, and on a level with the plane of the object stage. 



Fig. 123. 



Fig. 123 shows the first mctliod. A small rectangular segment of 

 II sph(!ricul shell has thrdc ditTcniit sized diaphragms cut, and the 



VOL. III. 4 A 



