HAND AND OTHER APPARATUS. 



47 



be, it will be found more satisfactory to have the 

 focussing under our own control. 



This method is a modification of that adopted 

 by Mr. Woodward when photographing without a 

 camera, under which circumstances we never use it, 

 having found the following arrangement much more 

 satisfactory. 



A cork with a slight rod attached to it by two 

 wire holdfasts, is slipped into a tube fastened to the 

 fine adjustment of the microscope ; this enables us to 

 act directly on the fine adjustment without the in- 

 tervention of any cord or wheels; unfortunately its 

 use is limited to those methods in which the micro- 

 scope is not attached to the camera. 



When sunlight is the source of illumination, there 

 is great danger of the object and objective being de- 

 stroyed by the heat rays focussed on them by the 

 condenser. This may be partially avoided by put- 

 ting the condenser a little out of focus, or we may 

 entirely eliminate them by the interposition of a plate 

 of alum between the heliostat and condenser. 



The best method, however, is to remove the blue 

 glass, and place a cell having parallel sides, and filled 

 with a solution of ammonia sulphate of copper in a 

 similar position to that recommended for the alum 

 plate. The cell is made as follows : — 



Fig. 20. 



