•.9> 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



the iluiiuuul fur au iucxpcnsivo form of rotating stage with iiioaus for 

 centering the motion exactly in the optic axis. The upper phite is 

 fitted to rotate on the lower one, and the latter is attaelied to the 

 Htandard or limb by the exccntric opening Khown. Tlie screws 

 enable it to be adjusted so tliat the rotation of the upper plate will 

 be concentric witli the optic axis. 



Fw. 110. 



Fig. 111. 



Such a system of centering must, we think, render the attachment 

 of the stage too unstable, which probably accounts for the fact of 

 Mr. Tolles not having further perfected it. 



Diaphragms for Beck's Vertical Illuminator. — The diaphragms 

 suggested by Mr. Tiglilmann * for the vertical illuminator were 

 found inconvenient in use as the force re- 

 quired to revolve the ring which carried them 

 rendered the apparatus liable to displacement. 

 Messrs. Beck now supply the illuminator 

 with the diaphragm shown in fig. 111. There 

 are two apertures, the smaller one being circular 

 and the larger one the shape of a broad 

 crescent. The latter gives many varieties of 

 form when moved laterally in front of the fixed 

 circular o] ening in the cylinder. 



In using the vertical illuminator with such 

 objects as A. peUucida, we have obtained the 

 best results by applying a narrow slot diaphragm to one of the small 



See this Journal, i. (1881) p. 041. 



