ARRANGEMENT OF APPARATUS. 123 



forms recently made, but from a consideration of the 

 character of the light required, we have some hesita- 

 tion in recommending them. 



One or two cautions are necessary with regard to 

 the batteries, and lamp. Bichromate cells are useless 

 for many reasons, but principally for not being suffi- 

 ciently constant. Somes means of regulating the 

 current is useful, as the sudden passage of a strong 

 current is liable to cause the lamp to burst. The 

 ordinary method of attaching the lamp by a spring 

 is most objectionable, as the slightest movement on 

 the same floor or adjoining apartment, will cause 

 sufficient tremor to utterly ruin the resulting nega- 

 tive. 



The whole stand is now raised to the perpen- 

 dicular ; the microscope stage accurately levelled, 

 and supported in that position by lengthening the 

 camera legs. 



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 hole on the 

 wheel diaphragm. The latter being approximately 

 centred 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- 



