CHAPTER VII. 



FOCUSSING SCREENS. FINE FOCUSSING WITH HIGH 

 POWERS. TRYING EFFECTS. OIL IMMERSION 

 LENSES. TEST OBJECTS. DIATOMS AS A 

 HOBBY. MEASURING THE AMPLIFICATION 

 WITH ANY EXTENSION OF CAMERA. TABLE 

 OF APERTURES AND MAGNIFYING POWERS 

 OF OBJECTIVES WITH EYEPIECES. THE 

 POLARISCOPE. COLOUR SCREENS. MONO- 

 CHROMATIC LIGHT. COLOUR PHOTOGRAPHY 

 (AUTOCHROME). 



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- 



