242 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



"We come now to the low powers (1 in. and 2 in., etc.) For these 

 two lenses we employ — ■ 



IlluminaUng System No. 2. — 1. For the 1-in. lens, with which we 

 use as condenser Watson's parachromatic divy condenser with front re- 

 moved (0-35 N.A.) : — 



{a) We adjust the Nelson lens as a collimating lens — i.e. so as to give 

 nearly parallel rays. This requires its being brought nearer the lamp 



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Fig. 37. 



than in the normal method, and may be roughly done by throwing a beam 

 of light, by means of an inclined plate of glass, so as to give a sharp 

 image of the lamp filament on the roof. 



(&) Centre it as before. 



(c) Now interpose, at about 7 in. behind the S.S. iris, a bi-convex lens 

 of about 7 in. focus, and centre it on the Nelson iris, as was done in the 



