ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



641 



were used. The object is, therefore, first found and centred with a low- 

 power visual lens, using the magnesium blue light. Then the high- 

 power monochrornat is used and the spot found which it is desired to 

 photograph. The camera is then moved into place, and the objective 

 must be adjusted for the change from magnesium to cadmium light ; 

 this adjustment must be determined by trial and noted for future use. 

 By making the arms of the magnesium electrode 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 focus at the same distance 

 beyond the prisms and the 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. It is found very advantageous to be 

 able to do all the exploratory and focusing work with blue light and 

 then to apply the ultra-violet light merely for the few seconds necessary 

 for photography. Moreover, owing to the greater precision in focusing, 

 it will be no longer necessary for the biologist to equip himself with a 

 whole series of expensive monochromats. 



Reichert's Movable Mechanical Object-stages.* — One type of these 

 auxiliaries is shown in fig. 141. The two adjacent rack screw-heads 



Fig. 141. 



a a' act in such a way that the object can be easily and safely moved in 

 two mutually perpendicular directions. Both co-ordinate edges are 

 graduated and provided with verniers for convenience of orientation and 

 for recovery of known positions. The circular periphery is radially 

 divided. By lifting up the screw b the centre object-holder can be re- 

 moved, so as to admit of the insertion of culture dishes. This stage is 

 only applicable to stands A I and A Ic. 



* C. Reichert, Vienna, Catalogue, Mikroscope.No. 26(1908) pp. 42-3, figs, 30, 32, 33 

 Oct. 21st, 1908 2 u 



