42 



HAND AND OTHER APPARATUS. 



and pinion or screw motion, and. when the desired 

 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 considera- 

 ble 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. 



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 

 surface 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 

 been found, of sufficient surface opacity and fine- 

 ness of texture, to take the place of patent plate. 



The second may be remedied by magnifying the 

 image. A focussing glass answers this purpose, an 

 excellent one being made as follows : — 



With a piece of stout cardboard make a tube four 

 or five inches long to fit one of the eye-pieces of the 



