124 ARRANGEMENT OF APPARATUS. 



tral with the pin hole cap, by bringing it into focus, 

 and displacing the wheel diaphragm sufficiently. 

 The low power objective is now racked back, and the 

 quarter inch substituted. 



The pin hole cap is removed and the perforated 

 disc brought into focus ; if not exactly central it is 

 made so by the centring screws fitted in the rim of 

 the diaphragm tube. 



At this stage in the arrangement of the apparatus 

 it is well to consider a peculiar method of illumina- 

 tion first described by Dr. Dallinger in the Micros- 

 copical Journal, and I believe called by him the sun 

 light. The following description will we hope ena- 

 ble others to obtain it. Unfortunately so much is 

 still left to individual intuitive skill, owing to no fixed 

 rules being found by which we might ascertain the 

 relative proportions of diaphragm aperture, power 

 and NA of condenser, intensity of illumination, and 

 other factors, that its accomplishment will at first be 

 found difficult. 



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

 forated disc is brought into focus with the quarter 

 inch objective and also centred. This must be done 

 by the centring 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- 



