CHAPTER VII. 



FOCUSSING SCREENS TRYING EFFECTS OIL IM- 

 MERSION LENSES TEST OBJECTS MEASUR- 

 ING THE AMPLIFICATION WITH ANY EXTEN- 

 SION 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 trans- 

 parent circles (about fin. diameter) in the centre 

 and four corners, made by cementing thin micro- 

 scopic cover glasses on with Canada balsam. 

 These will answer all purposes. Some beginners 

 work with a focussing 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 adjust- 

 ments and for moving the mechanical stage and 

 iris diaphragm, all of which may be easily reached. 



