Io4 ARRANGEMENT OF APPARATUS. 



exceeds three feet. It should also be remembered 

 that the actinic power of the light is said to decrease 

 as the square of the distance of the plate from the 

 source of illumination, and the duration of the ex- 

 posure varies inversely 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 orifice 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 convenient 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 sensi- 

 tive 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- 

 trance of actinic light. The subsequent manipula- 

 tions are identical with those already described. 



The following notes of the manipulations employed 



