HIGH-SPEED PHOTOGRAPHY 



767 



rotating prisms. Some of these are illustrated in Fig. 4. A comprehensive treatment 

 of these and other moving-optical systems as applied to projectors has been made by 

 Tuttle and Reed. 



The rotating-prism type is available from the Eastman Kodak Co., Rochester, 

 N. Y. The Carl Zeiss -Company, Jena, Germany, manufactures a camera in the 

 second classification. Cameras of the moving-lens type are made by the AUgemeine 

 Elektrische Gesellschaft, Berlin, Germany, and by the Merlin and Gevin Company, 

 Grenoble, France. 



In high-speed cameras employing stroboscopic light, the film is moved past the 

 lens at a constant speed; and each time the film has moved the distance occupied by 

 one frame, the subject is illuminated by a short intense flash of light. The time at 

 which the flash occurs is in some cases controlled by a contact disk rigidly attached 

 to the film-driving mechanism and properly spaced so that the motion-picture film is 

 properly framed for projection in standard equipment. Normal illumination such 

 as that encountered indoors is insufficient to fog the film in a stroboscopic-light camera 

 because the film passes the lens so rapidly. A stroboscopic-light camera is manu- 

 factured by the General Radio Company, Cambridge, Mass. 



When motion pictures are taken at high speed with either type of camera,^ the film 

 must move rapidly, and one of the important problems in the design is to make the 

 film travel at the requisite speed without vibrating, fluttering, or breaking. The 

 rapidly moving film must be guided properly, but the friction in sliding contacts may 

 generate enough heat to ignite it. Static charges of electricity resulting from the 

 friction must also be avoided, as they cause dendriform exposures on the film. Fur- 

 ther than simply traveling smoothly at a high speed, it is important that the film 

 accelerate rapidly so that it will attain the proper speed before much of it has passed 

 through the camera. The acceleration must be uniform as well as rapid, as sudden 

 jerks are likely to break the film. 



Table I. — Speed of Film in Terms op Frame Height and Rate of Exposure 



" Standard 35-mm. frame height. 

 '' Standard 16-mm. frame height. 



The height of the frame, as well as the rate of making exposures, is a factor influenc- 

 ing the film speed, since it is not necessary for the film to move as rapidly for a small 

 frame as for a large one. Most of the very high-speed pictures are small in size, in 

 some cases so minute as to be of little use in presenting detail even after enlargement. 

 Conversely, if the film speed is increased in order to produce larger pictures, the camera 

 becomes so bulky that it is no longer portable, and the subjects must be brought to 



1 Possible exceptions are those high-speed cameras employing stationary film and a large number of 

 lenses or a number of spark sources of light. 



