42 THE CAMERA, ETC. 



position is obtained, some means of fastening it is 

 necessary. 



The plate carrier or dry back should be easily 

 substituted for the focussing screen, as the slightest 

 shake might destroy an arrangement of the micro- 

 scope and its accessories, that consumed a consider- 

 able time in preparation. 



The camera, when these additions are completed, 

 is fastened to an oblong board, the same width as 

 itself, but projecting about two inches beyond it, 

 both back and front. To the corners of this board 

 levelling screws are attached which enable the 

 focussing screen to be made coincident with the 

 optic axis of the microscope, and the tube on the 

 bellows to be placed in the most favourable position 

 for adjustment to that instrument. (See fig. 24). 



The greatest difficulty that besets us, especially 

 when using artificial light, is the impossibility of 

 focussing minute markings on the screen.* This is 

 due to three causes, first the coarseness of the sur- 

 face on which the image is formed, second the 

 minuteness of the image itself, and thirdly want of 

 light. The first is irremediable as nothing has yet 



* A white card has been suggested as a substitute for patent 

 plate, it is not recommended for the following reason : our 

 aim being to photograph with high powers (anyone can photo- 

 graph with the |) and lamp light, the adoption of a white card 

 prevents the use of a focussing glass which is indispensable to 

 the resolution of minute markings, and also very sensibly in- 

 creases the illumination and renders faint shadows visible which 

 are invisible to the naked eye owing to the weak illumination. 



