Examination of Photographic Lenses at Kew. 

 FIG. 21. 



445 



FIG. 22. 



direction or the other, and this, therefore, is the position that the 

 photographer naturally adopts as his focus. By similar triangles 



a'fe'/AB = FaC/FaN! and FaC/CNx = 

 Therefore, since CNi and F 2 N! are approximately equal, 



a'V = AB . 



, = ~, cos 



Now FiF 2 represents 7, the movement of the ground glass, which was- 

 the measurement recorded. In the case of the examination for achrom- 

 atism, it was shown that it was unfair to conduct the test on a near 



