DELINEATION OF MICROSCOPIC OBJECTS. 



203 



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camera (see Fig. 24) that I have found most 

 convenient has the front which carries the lens, 

 etc., and the back which carries the prepared 

 plate, connected by a light-proof, bellows- 

 like body, which permits the back to slide 

 to and fro on a rigid base-board, about 

 4 feet 6 inches long. With this, any position 

 between one of 6 inches and 54 inches from 

 the lens can be readily obtained, and when 

 =^ obtained may be permanently secured by 

 a set-screw. In the front of the camera 

 is an arrangement for carrying the lens 

 and the object; the object is fixed on the 

 mechanical stage, and the image is formed 

 on the ground glass by moving the lens 

 by means of a rack and pinion, until the 

 best possible focus has been obtained. And 

 as, when the focus had to be ascertained, at so 

 great a distance as 4 feet 6 inches, it would 

 be manifestly impracticable to manipulate 

 this rack and pinion, they are actuated by 

 a pair of small bevelled wheels, the axis of 

 one of the wheels extending into a long 

 handle that lies along the side of the camera, 

 and is thus always within reach of the ope- 

 rator. The sensitive plate is contained in an 

 ordinary dark-slide. 



Third, as regards the lenses, my own expe- 

 rience has been, that the lenses as at present 

 constructed give the visual and actinic foci 

 coincident; but, no doubt, many good lenses 

 require correction; and this can be done, either 

 by putting the objective out from the object, 

 or by bringing the dark-slide nearer to it. If 

 this correction is needed, its extent can only 

 be found by experiment, and it will be at- 

 tained more easily by moving the dark-- 

 slide rather than the lens. An exceedingly 

 minute alteration of the latter makes a very 

 great difference; for, in using a long focal 

 distance, as above arranged, a large change in 

 the position of the sensitive plate — say, two 

 or three inches — will have the same result as 

 an extremely minute one in that of the lens ; 



