102 TRANSACTIONS OF THE [JAN. 16, 
and then the fork being caused to vibrate by a violin bow, the 
spot of light swings in a vertical line, H, which is impressed 
upon the film. The shutter is then released by the catch /, and, 
as it flies along its course, the vibrating spot of light traces a sinu- 
ous line upon the film, until the shutter reaches the end of its ex- 
cursion, when a second vertical line is traced. When the film 
is developed these lines appear, defined with an accuracy which 
depends upon the smallness of the spot of light and the care 
taken in performing the experiment. 
The rate of the fork being known, both the velocity and rate 
of progression of the shutter may be directly determined from 
the number and proportionate width of the complete undulations 
between the two vertical lines, and the measured distance 
actually traversed by the shutter. The latter would corre- 
spond to the distance between the two vertical lines, but for a 
slight expansion of the film, which sometimes occurs during 
development, and may increase their separation. The dis- 
crepancy is so slight that it may be almost disregarded; more- 
over the correction for it is very simple, and even this may be 
avoided, by the use of a thin plate instead of a film. 
Over a year ago I made a shutter to which was affixed a plate- 
holder for holding a sensitive plate, and I tried to employ a 
lamp as the source of light. But the shutter was too heavily 
weighted by the plate, and the lamplight spot made no im- 
pression on the plate. Recently, I was presented with a supply 
of the French ‘‘ Perron” films, and having provided means of 
using a beam of sunlight, I soon attained successful results. I 
find that a spot of quite intense sunlight is required, because its 
actual duration upon the film at any one point during the 
transit of the shutter is very brief, yet, I have no doubt, a 
lamp will be eventually employed. ‘The film referred to is so 
light that its weight may be disregarded. 
Fig. 2 represents a film thus treated and developed. It 
Fig. 2. 
moved with the shutter from left to right. Counting as one 
vibration each undulation to and fro of the sinuous line, the 
number found between the vertical lines is about seven. The 
fork used was an E fork, a shade flat, and its normal rate 1s 
not accurately known, but it was rated by the maker at 
331.25 vibrations per second, when loaded with about 1032 
