120 ARRANGEMENT OF APPARATUS. 



when considered as pointing from the periphery to the 

 central hole, it indicates directly the position of the 

 lamp and bull's eye with regard to the optic axis of 

 the instrument. At the same time the distance of 

 the bull's eye from the lamp may be slightly altered, 

 care being taken to keep it central. When all other 

 means have failed, a very slight alteration of the 

 position of the condenser by the centring screws, 

 will sometimes cause its almost magical appearance. 

 For further details we refer the reader to Dr. 

 Dallinger's communication. 



Having obtained the sunlight by some means or 

 another, we rack back the objective. Now if the 

 object is flat with fine markings on or near its sur- 

 face, we place it on the stage, but should it consist of 

 delicate scarcely differentiated structure, lying on dif- 

 ferent planes, we replace the pin hole cap, on the top 

 of the condenser before placing the object on the 

 stage. The pin-hole cap should itself be accu- 

 rately centred, and if not, rejected and another 

 tried. With the objective corrected for uncovered, 

 (the ultimate correction for thickness of cover 

 glass being made according to the method de- 

 scribed on page 17, chap, ii.) we bring the 

 object into focus. If the light is too great or 

 deficient, the condenser is slightly moved farther 

 from, or nearer to the object, or the inner tube con- 

 taining the perforated disc may be removed and an- 

 other substituted, and the tube returned to exactly 

 the same position, care being taken to avoid disturb- 

 ing the relative positions of any of the other appar- 

 atus. During its slow removal the eye should be 



