ARRANGEMENT OF APPARATUS, ETC. 113 



The lamp and bull's eye being centered, the per- 

 forated disc is brought into focus with the quarter 

 inch objective and also centred. This must be done 

 by the centering screws, to prevent disturbance of 

 the relative positions of the substage apparatus. 

 The aperture in the disc, when the sunlight is ob- 

 tained, should appear as an intensely bright spot in 

 the centre of an illuminated field A, fig. 27. 



The chances are a hundred to one that it appears 

 as represented at either B or C. If as at C it shows 

 that either the lamp or bull's eye condenser is en- 

 tirely out of the optic axis of the instrument ; if as 

 at B, one or other is too much to the left of the 



Fig. 27. 



instrument ; the condenser and diaphragm are sup- 

 posed to be already accurately centered, and in fact 

 must be if the foregoing directions have been pro- 

 perly carried out. 



Whichever appearance presents itself the bull's 

 eye condenser is turned to one side, and the image 

 of the lamp brought into focus. If the flame is not 

 central it is made so, by the rack and pinion, or 

 lateral screw motion attached to its stand. The 



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