112 ARRANGEMENT OF APPARATUS, ETC. 



The eyepiece tube being fitted to the microscope, 

 a one inch or lower power is screwed on, and the 

 condenser, with its pin hole cap, lamp, and bull's 

 eye centred as before. We remove the inner tube, 

 or central sliding diaphragm as it is called, from the 

 cloth lined tube fastened to the largest bole on the 

 wheel diaphragm. The latter being approximately 

 centered under the condenser, we insert the inner 

 tube fitted with a perforated disc, (the perforation 

 being about one-tenth of an inch in diameter or less) 

 till it touches the surface of the lowest combination 

 of the condenser. It is now made accurately cen- 

 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 centering 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. 



