122 ARRANGEMENT OF APPARATUS, ETC. 



The distance of the screen, owing to its not being 

 continuous with the microscope, is only limited by 

 the length of the dark room. Practically it will be 

 found that perfect results are not obtained when it ex- 

 ceeds three feet. It should be also remembered that 

 the actinic power of the light decreases as the square 

 of the distance of the plate from the source of illu- 

 mination, and the duration of the exposure varies in- 

 versely as the actinic power. 



Seating ourselves behind the camera and taking the 

 focussing rod, (which may be conveniently supported 

 within reach by a string) in our left hand, we apply 

 the focussing glass to the screen with the right. 

 When the proper focus is obtained the rod is let 

 gently down. The sulphate of copper cell or blue 

 glass is placed in position ; the windows closed or 

 sleeves fastened ; and the curtain which has an ori- 

 fice for the tube of the microscope only, allowed to 

 envelope the whole of the recess, except the tube. 



Two tightly stretched strings or laths, reaching 

 for about four or five feet beyond the camera and 

 about seven from the floor, will be found very con- 

 venient to hang a black cloth over, covering the 

 camera and making it practically continuous with 

 the microscope during exposure. The possibility of 

 any actinic light, except that from the microscope, 

 reaching the sensitive plate is thus prevented. 



A cap is placed over the orifice of the microscope 

 and we proceed to place the dry plate in the carrier. 

 Twenty times collodion should be used. Great care 

 must be taken to avoid shake and prevent the en- 



