CHAPTER V. 



FOCUSSING SCREENS TRYING EFFECTS OIL IM- 

 MERSION LENSES TEST OBJECTS MEASURING 

 THE AMPLIFICATION WITH ANY EXTENSION OF 

 CAMERA THE POLARISCOPE COLOUR SCREENS. 



Quite a chapter could be written on focussing 

 screens and the mode of preparing them. Fine 

 ground-glass screens can be bought with transparent 

 circles (about ^in. diameter) in the centre and four 

 corners, made by cementing thin microscopic cover 

 glasses on with Canada balsam. These will answer 

 all purposes. Some beginners work with a focus- 

 sing cloth, underneath which they get quite hot and 

 exhausted. If the gas in the room be turned down, 

 there is no necessity for a cloth, and focussing can 

 be carried on with comfort. It is well to sit down 

 to the work and take it easy ; standing and bending 

 the back are tiring. 



To focus, the focussing glass is held in one hand, 

 leaving the other hand free for focussing first with 

 the coarse and then with the fine adjustments ; and 

 for moving the mechanical stage and iris diaphragm, 

 all of which may be easily reached. The rough 

 focussing should be done through the eyepiece, 

 with the camera and sliding block removed, as ex- 

 plained previously. 



