112 TRANSACTIONS OF THE [JAN. 16, 
at a distance, of an object moving at greater velocity than the 
shutter, and which at a less distance would afford but a blurred 
image upon the plate. This leads, perhaps, to many illusions 
respecting the rapidity of so-called instantaneous exposures. 
To obtain an exposure of 4, of asecond, with an almost friction- 
less shutter weighing only 8 grammes, under the conditions de- 
scribed, a strain of about 250 grammes was required ; and to 
attain the ;4, of asecond the strain increased to about 719 
grammes, almost three times as much. ‘The proportional in- 
crease required to reach the 3,55 of a second, were such an ex- 
posure attainable, may be imagined. 
Since, with full sunshine entering the lens, however, the ef- 
fective is so much less than the actual exposure, the difference 
with the weaker light used in practice must be proportionally 
greater, and we must not lose sight of the probability that our 
measures correspond to much shorter effective exposures. 
The weather has afforded me no opportunity of taking test 
pictures with exposures specially measured by the method, but 
I have formerly taken many which compare favorably with those 
usually credited to very high exposures. Knowing nearly the 
tension used, I find by subsequent measurement that the shutter 
exposure employed was approximately the one hundred and 
seventieth of a second. ‘The example, Fig. 17, will serve to il- 
Fie. 17. 
lustrate the relation of this exposure to a moving object finely 
lighted. 
Should this method sustain a more critical study, and the ex- 
actions of scientific application, it would appear to be the hidden 
