92 THE A B C OF PHOTO - MICROGRAPHY 



limited solely by their length. If it be found 

 too great, the operations are to be reversed 

 the lens removed farther from the subject and 

 the bellows shortened. The desired magnifi- 

 cation having been attained, final focusing is 

 done by moving the screen back or forth, and 

 for such low powers the ground glass is suffi- 

 ciently fine to do so with the greatest accu- 

 racy. If our room has not been darkened, it 

 will be found that a large amount of light en- 

 tering the window falls upon the lens without 

 first passing through the object. This must 

 be cut off by a cloth or some other covering 

 placed over the space between the two. A 

 stop or diaphragm of about //22 must then be 

 inserted and cardboard or other device placed 

 in front of the object or between object and 

 lens, as may be found most convenient, to cut 

 off entrance of light whilst the slide is being 

 drawn from the holder. If the lens is pro- 

 vided with a time shutter so much the better, 

 but cap exposures are not advisable, for fear 

 of disarranging something. The length of 

 exposure can only be ascertained by practice 

 and experiment, but something may be learned 

 about it by carefully studying the explanations 

 as to the making of the four negatives herewith 

 reproduced. Should artificial illumination be 



