4 PHOTOGRAPHIC INTRODUCTION 



no ultimate structure comparable to the fine details 

 of the diatom, it is a disadvantage to employ more 

 colour than is absolutely necessary to secure the 

 requisite contrast, as this merely increases the 

 exposure without adding to the perfection of the 

 final result. Screens of all colours were employed to 

 secure contrast only; the one most commonly used 

 was a screen of " pot-green " glass of medium density 

 and -J inch thick. This appeared to be the most 

 suitable for the majority of stains which we met 

 with in the specimens. 



Focussing in photomicrography is always a diffi- 

 culty. Sometimes an ordinary ground glass was 

 used, and at others an exceedingly fine variety 

 made especially for us by Ross & Co. A Dallmeyer 

 hand magnifier was used with this. 



The disadvantage of using the finest ground glass 

 is that the general lighting of the field is entirely 

 lost, while, with the rougher variety, though the 

 general field of view is well seen, the coarse grinding 

 prevents accurate focussing without some diffi- 

 culty, especially when the hand magnifier is used. 

 The focussing screen in the present instance was 

 so constructed that the glasses could be readily 

 changed as thought best. 



An even illumination of the field is important if 



