766 



HANDBOOK OF PHOTOGRAPHY 



source of light actuated by a condenser discharge. While only one photograph at a 

 time can be taken bj^ this method, in contrast to a large number by the high-speed 

 motion-picture camera, many practical problems can be solved by this method, par- 

 ticularly if the action of the subject can be repeated. 



High-speed Motion-picture Cameras. — Motion pictures taken at high speed and 

 subsequently shown at ordinary projection speeds have the effect of slowing fast 

 motions so that the eye can see what is going on. Naturally the faster the action 

 that is being photographed, the faster the camera must be driven. It seems imprac- 

 tical, because of mechanical limitations, to drive the usual intermittent types of 

 motion-picture cameras at speeds in excess of about ten times normal. Practically 

 all cameras operating above this speed depart from the intermittent-motion mecha- 



Objecf 



.Film 



Active ' yo" 

 lens 



-j-l^ Shaft gearecl to sprocket 

 -:/- Lenses 



Moving Lens Method 



'Film on inner rim of drum 



Mu/tisioleol mirror geared 

 to motion of film drum 



-'-Cross section ofolrum 



Moving Mirror Method 



lens 



\ Film 



Prism ro tated by 

 p . gear connection to 



frism film driving sprocket 



Ro+Ofting Plane Prism Me+hod 



Flu. 4. — Diagram showing various possible optical systems for use in high-speed motion- 

 picture cameras. 



nism and use instead a continuously moving-film mechanism. Although the film 

 moves continuoush' during the exposure, the image cast by the lens on the film must 

 either move with the film or the film must not move an appreciable distance during the 

 exposure time. The first requirement is met by the use of a moving optical system to 

 keep the image stationarj^ with respect to the moving film during the exposure time; 

 and the second, by the use of intermittent illumination (stroboscopic light) for exposing 

 the photographs, each flash of light lasting such a short time that sharp images are 

 obtained although the film is moving. The moving-optical-system method is espe- 

 cially well adapted to the study of subjects which emit their own light, common exam- 

 ples of which are the burning of vapors, the action of explosives, the motions of an 

 electric arc, the reactions in a photoflash lamp, and the behavior of the cathode spot 

 in a mercury-arc tube. 



There are three general types of the moving-optical-system camera: (1) those 

 using rotating lenses or slits, (2) those using rotating mirrors, and (3) those using 



