1888. | NEW YORK ACADEMY OF SCIENCES. 105 
Fig. 6, is obtained, affording 4 vibrations between the vertical 
lines, and 24 undulations within the intermediate 3.81 cm., 
which indicates an exposure of ;4, of a second, during which 
the train would advance 10.4 cm., and a velocity for the 
shutter of 3.933 metres per second, which is about 4 the ve- 
Fia. 6. 
locity of the train. Other cases can be estimated quite as 
easily. 
Taubes cases the distinction between slow, medium, and 
quick is that which is customary in the actual use of the shut- 
ter employed in these experiments. ‘The shutter is, however, 
not uncommonly driven at a greater speed than that indi- 
cated in the latter of these cases, in which, as being selected at 
random simply to illustrate the method and not for the com- 
parison of exposures, other details are omitted. 
The relative velocities obtained by springs of different ten- 
sions and kinds, and by shutters of various designs, are now the 
subject of investigation, and will be given in a future paper, but 
to other indications of the experiments already tried we may now 
advert. 
I have long been satisfied that the motion of a shutter ac- 
y 
Fia. 7. 
tuated by a retractile spring is accelerated like that of a falling 
body, but in greater proportion, and each graphic record of the 
shutter’s motion infallibly confirms my deduction. In Fig. 7 A, 
