1888. NEW YORK ACADEMY OF SCIENCES, abla 
defeat the latter purpose, the film may again be advantageously 
employed. 
The entire avoidance of friction seems to render this method 
applicable as a new means of measurement, in many cases 
other than those relating to photography. It is hoped by its 
means to measure the duration of electric and lightning dis- 
Fie. 16, 
No 
charges, and the flash given by various illuminants, such as 
magnesium powder and gun cotton. 
The beam of light, however, must be adapted to the kind of 
shutters examined. Ina central opening shutter, for instance, 
only a minute pencil of light entering the centre is necessary. 
But with an ordinary drop shutter, if such a pencil be used, it 
gives only the time occupied by the opening of the shutter in 
passing the point where the ray enters, irrespective of the 
size of the exposing aperture. In this case, the exposure may 
be accurately obtained by calculation ; but if a converging beam 
of light, filling the exposing aperture, be employed, the undula- 
tions obtained would probably afford a close approximation to 
the true time of exposure. With the assistance of this method, 
every shutter might be marked by the maker with a sequence 
of exposures, corresponding to certain tensions of the spring, 
and afford an additional guide and gratification to the user. 
Exposures varying from the 31, to z,/5 part of a second are 
not uncommonly mentioned. Mr. Edward Maybridge, in his 
circular of February, 1884, promised that verified exposures of 
3000 part of a second would be used. Some relation of the ve- 
locity of the shutter to the angular velocity of the moving object, 
accounts for the apparently sharp pictures that may be taken 
