1888. | NEW YORK ACADEMY OF SCIENCES. 101 
a method which may perhaps become available. In adopting as 
a measure the vibrations of a tuning-fork it resembles the 
method of Mr. Albert Londe,’ from which it differs in dis- 
pensing with a standard source of light, and in avoiding the 
complications arising from the individual equation of lenses, by 
determining directly the speed and rate of translation of the 
shutter, and therefrom the shutter exposure, which is a positive 
element of quick exposures. 
By acombination of the two methods it would appear that 
the essential elements of an instantaneous exposure may be quite 
accurately determined, for while Londe’s method deals with the 
lens, mine more particularly relates to the shutter. Certain 
principles which appear to be common to a large variety of 
shutters have been established by the first application of the 
method to the ordinary guillotine shutter. The term guillotine 
shutter, borrowed from the French, is herein used to designate 
any sliding shutter actuated by gravitation or a spring, whether 
it move downward, sideways, or upward. 
Instead of the elaborate apparatus of Londe, a beam of sun- 
light, a tuning-fork, and a lens, constituted the easily arranged 
apparatus I first employed, which may, doubtless, be reduced to 
a portable form of such simplicity that any one may use it at a 
Fic. 1. 
moment’s notice. By means of the diagram, Fig. 1, it may be 
explained as follows: 
By reflection from a mirror, B, fixed upon one arm of a 
tuning-fork, a beam of sunlight, A, from a heliostat, is directed 
in a dark room through a meniscus, C, which projects it as a 
minute but brilliant spot, upon the shutter, D, of a camera, LZ. 
A counterpoise to the mirror must be fixed upon the opposite 
arm of the fork, and then its rate be determined. 
The method as conducted with the apparatus is as follows: 
Upon the shutter D a light sensitive plate or film is fastened, 
1 Albert Londe, La Nature, May 19 and 24, 1887. 
