Vol. 6, 192a PHYSICS: THOMPSON, HICKMAN, RIFFOLT 
171 
trical hammer and allowed to vibrate freely, and in so doing causes the image 
of the slit in front of the arc to move up and down across the stationary slit. 
The stationary slit, then, is an intermittent source of light, the image of which 
is focused at a convenient place on the moving film. The optical system 
should be so designed that the beam only slightly more than covers the 
mirror. A new fork has been constructed giving about two thousand lines 
•per second. There is no reason why this should not be as high^as^ten'to 
BhU/ST/C APPARATUS 
twelve thousand lines per second. The velocity at the neutral position 
(where the slit is placed) is a maximum and includes the frequency as a 
factor. As it is increased the amplitude may decrease and still permit 
the same exposure of light through the slit as at the lower frequencies. 
Mechanically, the essential condition to be fulfilled in the design is that 
the natural frequency of the filament upon which the mirror is mounted 
should be high in comparison with that of the fork. 
Most of the methods which are at present used in ballistics for the 
measurement of velocity involve the passage of the projectile through 
screens of one sort or another, breaking or making electrical circuits. 
The method which we have devised does not require that any material be 
touched by the projectile in its flight, but simply the passage twice through 
a beam of light. It is found that it is only necessary to observe the flight 
of the projectile over a space of from 3 to 5 feet to get a precision 
which is certainly greater than that needed for ordinary purposes. 
The beam of light obtained from a slit is controlled by the optical system 
(fig. 2) in such a way that it is focused twice, each time in the path of the 
projectile, being reflected by the mirrors Mi, Mi and M3 and finally brought 
to the photographic surface on the rotating drum. The image is a short 
sharp line. It is desirable to have for both this and the calibrating sys- 
tem a short focus lens. As the film rotates, the image of the slit will 
