1888. ] NEW YORK ACADEMY OF SCIENCES. 10? 
at its termination. This is caused by a rebound of the shutter, 
although it struck against a felt stop. This rebound could not 
occur in the three cases first presented, because a spring catch 
prevented it. This catch I have used for a long time in a hand 
camera, especially when making rapid exposures, because I 
found that in such cases the rebound might be sufficiently great 
Fie. 8. 
W/V 
to cause a partial reopening of the aperture, and a double ex- 
posure. 
Through the maze of its vagaries we can, however, easily fol- 
low the primary line. This is one of four exposures made at 
different times, with considerable intervals between. Theshutter 
opening, and exposing aperture, were about the sameas before, 
namely, 1.9 cm. in diameter. In all the films, as in this, there 
are about 44 undulations between the vertical lines. All these 
films expanded in different degrees, but, allowing for the expan- 
sion in each case, there are about 34 undulations in the middle 
3.81 cm. From this it appears that the rubber springs do not 
change materially during short intervals. 
These latter experiments were made with a fork of unde- 
determined rate, loaded with a much larger mirror than that 
formerly used. The camera with which they were made was so 
constructed that two shutters, very nearly alike, one in the 
usual place, the other in the focus of the lens of the camera where 
the plate should register, are simultaneously released, and being 
actuated by similar springs, stretched to the same extent, cross 
the camera in very nearly, if not exactly, the same time. The 
four exposures last referred to were made with films upon the 
front shutter, in the manner previously described; but in con- 
nection with two of them, one of which is the one shown, sup- 
plementary exposures were made, to which a moment’s atten- 
tion may be given. For each of these the camera was relocated, 
so that the beam from the mirror on the fork fully covered the 
opening of the lens, which focussed it toa point on the inner shut- 
ter. Upon this, in each case, a film was then fastened, and an ex- 
posure made while the fork was vibrating, both shutters moving 
together, but no film being upon the front shutter. These ex- 
posures were made in reality by the front shutter; the motion 
of the back shutter, which merely served to draw out the wave 
