STEREOSCOPIC WORK 329 
if two separate similar cameras are used together, measure N 
with negatives placed edge to edge, and in the same relative 
positions that they occupied during exposure; then 
Make P = E — WN -t length of one plate. 
If a prismatic stereoscope, fitted with properly centred half 
lenses, is used, add the width of one prism to above values 
Or 7. 
Frequently, if the prints are cut apart, errors may arise, 
and they may be wrongly mounted. To avoid this, place the 
prints face down on the table, with the top away from you, 
and across the central dividing line draw a faint pencil line, 
about one inch long. Then cut the prints in two, and, after 
trimming, mount them so the pencil lines fall on the outside 
margins of the prints. 
In order that the picture shall be viewed as through a dark 
frame, which is the most effective way, trim the prints as 
follows: the prints should be 214 inches wide, but mounted 
with an eighth of an inch between them, and trimmed so 
that there is one-eighth of an inch more subject shown on 
the right-hand side of the left-eye print than on the right 
of the right-eye print, and one-eighth of an inch more subject 
shown on the left of the right-eye print than on the left of 
the left-eye print. 
