478 PROCEEDINGS OF THE AMERICAN ACADEMY 



Investigations on Light and Heat made and puBi.isnED wnoLLT ok in part with 

 Appropriation from the Kumford Kund. 



XXIV. 



CONTRIBUTIONS FROM THE PHYSICAL DEPARTMENT OF THE 

 MASSACHUSETTS INSTITUTE OF TECHNOLOGY. 



XVIII. — METHODS OF DETERMINING THE SPEED 

 OF PHOTOGRAPHIC EXPOSERS. 



By William II. Pickering. 



CommunicateJ January 14, 1885. 



One of the best-known methods for this determination depends on 

 photographing a white clock hand, which revolves rapidly in front of 

 a black dial. The chief difficulty in this case is to maintain a uniform 

 rotation at high speed. To avoid this difficulty a method was sug- 

 gested by me in " Science," Nov. 14, 1884, which depended on photo- 

 graphing a spot of light reflected in a mirror attached to a vibrating 

 tuning-fork. Tiie objection to this plan is that most photographers 

 cannot readily obtain a proper tuning-fork and measure its pitch. 

 Two other methods have therefore been devised in which this difficulty 

 is obviated. 



We sometimes see it suggested that, where a true " drop shutter " is 

 used, we may determine its speed by the well-known laws of falling 

 bodies. That this method, if adopted, would give only approximately 

 correct results, is illustrated by the following experiment made with an 

 exposer in which the grooves were of wood, and the shutter itself of 

 hard rubber. The shutter measured 100 cm. (40 inches), in length in 

 order that high speeds might be obtained. It fell apparently perfectly 

 freely, and the friction was reduced to a minimum. The theoretical 

 and measured lengths of exposure are given below, the measurements 

 being made by the method about to be described. The first column 

 gives the distance that the shutter fell before the middle of its aperture 

 reached the middle of the aperture between the lenses. The second 

 column gives the theoretical exposure with a 2.0 cm. aperture; and 

 the third column gives the exposure as measured. 



