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SUMMARY OF CURRENT RESEARCHES RELATING TO 



screen lights 



up under the action of the ultra-violet rays, and focus- 

 ing is then possible. Focusing high-power monochromatic objectives 

 is, however, tedious and difficult ; but the authors consider that they 

 have devised a great improvement in this respect. Instead of employ- 

 ing a single pair of electrode holders, they recommend a double pair 

 (four in all) arranged so that the cadmium electrodes can be instantly 

 swung out and replaced by a pair of magnesium electrodes by means of 



Fig. 140. 



the handles shown in fig. 140. The cadmium eleotrode holders are 

 longer than those for the magnesium, for a purpose explained later. 

 There is an automatic stop on the lower pair of holders to insure the 

 spark gap falling in the axis of the collimator lens. The swing-out 

 electrode changer was suggested by the discovery that the monochromatic 

 lenses, though giving only badly blurred and coloured images with 

 ordinary light, did give very good images that could be focused sharply 

 even to the finest detail providing strictly monochromatic visible light 



