MEASURING UPON THE SCREEN. 69 
The writer is in the habit of making this record 
upon the right-hand side of the scantling upon 
which the camera slides, as follows, — 
gle gl2 s{f oe 
After making this determination for the one- 
inch alone, make it for the same lens in combina- 
tion with the amplifier; then for the one-fifth- 
inch; and finally, for this and the amplifier to- 
gether. 
The exact point is fixed in each instance by 
moving the screen forward or back until the lines 
are at the proper distance from each other, to give 
the desired amplification, This is determined 
most conveniently by a pair of dividers, which 
are adjusted from a scale of inches. Suppose, for 
example, that the points of the dividers are ad- 
justed to one inch apart. The screen is now 
moved to such a position that four of the one- 
hundredth-inch spaces (five lines), fall between 
the points of the dividers. The amplification will 
evidently be twenty-five diameters. Now move 
the screen farther back until two of these spaces 
fall between the points of the dividers, and we 
have fifty diameters, etc. It will be necessary 
carefully to focus the lines for each determination, 
and to place the points of the dividers precisely in 
the centre of the dividing lines. 
Having made this determination once, there 
will be no further trouble about measuring. It 
must be remembered, however, that the condi- 
