138 THE A B C OF PHOTO - MICROGRAPHY 



are probably familiar to the majority of my 

 readers, but at the risk of repeating an oft- 

 told tale I will give them once again. Let 

 us suppose that we have just made a nega- 

 tive under a low or medium power and wish 

 to know the number of diameters employed 

 in the making. Before changing the micro- 

 scope or camera in any manner, replace the 

 object with a stage micrometer ruled in parts 

 of an inch as above and focus the projected 

 lines sharply upon the ground-glass screen, 

 using those TO of an inch apart for this 

 moderate enlargement. Now, applying one 

 leg of the dividers to a line, we extend the 

 point of the other until it rests upon the 

 next adjacent line. Should the distance from 

 point to point be found to measure, say just 

 one inch upon the scale, it is obvious that 

 the enlargement is one hundred diameters, 

 each hundredth division of the micrometer 

 having been magnified to that extent. Of 

 course this enlargement is equal in every 

 direction, horizontal, vertical, diagonal, so 

 that in popular parlance it is really ten 

 thousand times, or the square of the diam- 

 eters ; the latter term alone being used scien- 

 tifically, as it is presumed my readers all 

 know quite well. Should the divider points 



