THE BIRD PIIOTOGRAPHI^R'.S OUTFIT 21 



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 ToVo 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 i)art uf 

 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 emyiloyed ; time exposui'es, 

 however, can not be made with the fncal-phme 

 shutter, and for all - around work the camera 

 should be fitted with both a lens and a focal-plane 

 shutter. 



The reflecting can"iera, 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 



