126 ARRANGEMENT OF APPARATUS,, ETC. 



Before concluding our remarks on this part of 

 of tbe subject, we would impress upon the photo- 

 grapher the necessity for determining the magni- 

 fying power in every case. This may be done as 

 follows: 



After removal of the carrier containing the sen- 

 sitised plate from the camera, an ordinary micro- 

 meter slide divided into one-hundroths and one-thou- 

 sandths of an inch is substituted for the object. Its 

 image is focussed on the ground glass, and the divi- 

 sion measured with small rule. For example if one 

 of the divisions corresponding to one-hundi*eth of an 

 inch on the micrometer measures two inches on the 

 ground glass, the magnification equals two hundred 

 diameters. 



It is not necessary to adopt this method in every 

 case, for, if the magnifying power of a certain objec- 

 tive is known when the image formed by it is at two 

 separate distances from any object, the magnifying 

 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 subsequent occa- 

 sion we are photographing with the same lens, and 

 the screw is twenty-four inches from the object, the 

 magnifying power would be one hundred and fifty 

 diameters. 



It is sometimes difficult to tell if the sun illu- 

 minates the object equally when a heliostat is not 



