182 



SCIENCE 



[N. S. Vol. XXVI. Xo. 658 



monocliromat is used and a detailed explora- 

 tion made of the object, using the blue light 

 all the time in a room lighted as much as 

 desired by incandescent lamps or otherwise 

 (the room should be darkened when the photo- 

 graphic exposure is made). Finally, when 

 a particular spot is found of which a photo- 

 graph is wanted, the camera is moved into 

 place and then all is ready for the exposure 

 except for a correction of the focus of the 

 objective due to the change in wave-length 

 from 448 /xfi to 275 fifi. This latter correction 

 must be worked out by trial for each objective, 

 but once determined can in future be made in 

 a moment. The objective when used with 

 ultra-violet light must be racked down a con- 



FlG. 1. Electrode holders supplied with the 

 ultra-violet microscope by the maker. Two simple 

 holders .with screw clamps to hold the wire (or 

 ribbon) electrodes. 



siderable distance below the focal point for 

 the blue rays. This distance that the object- 

 ive must be lowered is read off on the scale 

 of the fine adjustment screw of the micro- 

 scope stand. In case of our 1.7 mm. mono- 

 chromatic objective the focal correction 

 amounts to forty divisions of the fine adjust- 



ment screw of the Zeiss Photcimicrographic 

 stand (about 0.08 mm.). 



By having the arms of the magnesium elec- 

 trode holders 5.5 mm. shorter than those for 

 the cadmium it was found possible to bring 

 the blue light and the ultra-violet rays to a 



Fig. 2. New swing-out electrode holders. With 

 two pairs of holders; the short ones for mag- 

 nesium, the long for cadmium. The holders are 

 open above so the electrodes can be removed easily 

 for adjustment. The lower pair have a stop to 

 bring the electrodes automatically in line. Either 

 pair of holders may be thrown into position by 

 turning the handles. About three tenths natural 

 size. 



focus at the same distance beyond the prisms 

 and collector lens, though not in the same 

 spot, as the ultra-violet rays are refracted 

 much more than the blue rays in passing 

 through the prisms. The illuminating ap- 

 paratus is made to swing laterally as a whole. 



