68 PHOTO-MICROGRAPHS. 
stage-micrometer, ruled in fractions of an inch, and 
find that the lines one-hundredth of an inch apart 
are separated from each other by exactly one inch, 
it is evident that the amplification effected by the 
optical combination used, with the screen at this 
distance, is one hundred diameters. If the lines 
one thousandth of an inch apart on the microm- 
eter are one inch apart on the screen, the magni- 
fying power is one thousand diameters, etc. 
The exact amplification of every photo-micro- 
graph made should be recorded, and it is desir- 
able that the magnifying power employed be 
expressed in hundreds or even fractions of one 
hundred, as 500, 250, 50, 20, ete. For this purpose, 
and to save the labor of making measurements 
whenever a different objective is employed, or the 
position of the screen changed, the beginner is ad- 
vised, before making any attempt to photograph, 
to mark upon the scantling from which the camera 
is suspended, the position of the screen with an 
amplification of 100, 200, 300, ete. (or fractions 
of 100 with lower powers), for each objective or 
optical combination which he proposes to use. 
For example, if a one-inch, a one-fifth-inch, and 
an amplifier are the optical appliances to be used, 
the operator would first determine the position 
of the screen for the one-inch alone, marking by a 
perpendicular line the exact location of the screen 
when the one-hundredth-inch lines are one fourth 
inch apart, — twenty-five diameters; one half inch 
apart, — fifty diameters; one inch apart, —one 
hundred diameters, etc. 
