THE BIRD PHOTOGRAPHER’S OUTFIT 91 
power, but more particularly by decreasing the 
width of the slit. Assuming, therefore, that it 
takes one second for the slit to pass from top to bot- 
tom of a plate four inches high, and that the slit is 
one inch in width, it follows that each portion of 
the plate is exposed to the light for a quarter of a 
second. Decreasing the width of the slit one half, 
proportionally reduces the time of the exposure, 
and by this means, in connection with an increase in 
the speed with which the curtain is moved, an ex- 
posure of zyoo of a second is possible. 
In addition to possessing the advantage of great 
speed, this shutter also passes a higher percentage of 
light than a lens shutter even when the actual time 
of the so-called exposure is the same. This is due 
to the fact that the lens opening is in no way af- 
fected, it being the same throughout the exposure. 
With a lens shutter, on the contrary, the full value 
of the opening is given for only a fractional part of 
the exposure, the parts of the shutter more or less 
filling the opening during the rest of the time. 
With a focal-plane shutter, therefore, one may do 
rapid work under conditions where a lens shutter 
could not be successfully employed; time exposures, 
however, can not be made with the focal-plane 
shutter, and for all-around work the camera 
should be fitted with both a lens and a focal-plane 
shutter. 
The reflecting camera, as before stated, is fitted 
with a focal-plane shutter, and, as described, it is 
released by pressing the lever, which raises the mir- 
ror. Lens shutters, however, are released by a 
pneumatic bulb, or in some cases by a thread or 
