138 ARRANGEMENT OF APPARATUS. 



light into the dark room, and while placing the sen- 

 sitised plate in the carrier as well as during develop- 

 ment, no more light should come through the ruby 

 glass, or exposure of the plate to it be allowed, than 

 shall just enable us to work. 



Before concluding our remarks on this part of the 

 subject, we would impress upon the photographer 

 the necessity for determining the magnifying power 

 in every case. This may be done as follows : After 

 removal of the carrier containing the sensitised plate 

 from the camera, an ordinary micrometer slide di- 

 vided into one-hundredths and one- thousandths of an 

 inch is substituted for the object. Its image is 

 focussed on the ground glass, and the division 

 measured with small rule. For example if one of the 

 divisions corresponding to one-hundredth of an inch 

 on the micrometer measures two inches on the ground 

 glass, the magnification equals two hundred diame- 

 ters. It is not necessary to adopt this method in 

 every case, for, if the magnifying power of a certain 

 objective is known when the image formed by it is at 

 two separate distances from any object, the magnify- 

 ing power can be determined when used at any other 

 distance. Suppose the magnifying power with the 

 quarter inch six inches from the object, has been 

 found to be fifty diameters, and at two feet and a 

 half, to be two hundred diameters. If on a sub- 

 sequent occasion we are photographing with the 

 same lens, and the screw is twenty-four inches from 

 the object, the magnifying power would be one hun- 

 dred and fifty diameters. 



