ARRANGEMENT OF APPARATUS. 125 



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 centred, 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 

 position of its reflected image in the mirror behind it 

 should be observed, and if not coincident with the 

 flame itself, it is made so by tilting the tin shade sur- 

 rounding the lamp. The bull's eye is again brought 

 into view and centred. If the sun-light is not yet 

 perfect or if it appears as at B, the lamp is moved 

 more to the left or right according to the position of 

 the bright ray proceeding from the aperture of the 

 diaphragm. These manipulations are greatly facili- 

 tated by remembering the relation the bright ray 

 bears to the position of the lamp and bull's eye ; for, 



