HIGH-SPEED PHOTOGRAPHY 



763 



sensitive paper. A modification of the method is to substitute a ground glass for the 

 film and then photograph the image from the back with an ordinary camera. 



The second method illustrated in Fig. 1 uses a large condensing lens (not neces- 

 sarily of high optical quality) between the subject and the light source. Adjustments 

 are made so that the image of the light source appears on the film as a circle large 

 enough to cover the film. The camera lens is then focused on the subject (using 

 reflected light), and a shadow of the subject appears on the film, reduced in size opti- 

 cally as in ordinary photographic procedure. The aperture is very small, since it is 

 determined by the dimensions of the light source, and therefore the depth of focus is 



Fi/m or 

 paper 



Focus 

 fens on 

 fob/'ecf 



'Ac/jusf conc/enser unfi/ field is^^ 

 filled with light in camera -' 



lig/it 

 H Shcidovy 



Spark Stop 

 1. 



Lens 



^ecf stop 



HI Schlieren Method of Toepler 

 - Sphenccr/ mirror 



Cornbinafion ^' 

 mirror and siop 



Camera 

 Spark 



12" Schlieren 

 Fig. 1. — Diagram illustrating various methods of making high-speed silhouette photo- 

 graphs. 



great. Two advantages result from the use of this arrangement, viz., (1) the effective 

 use of the light, since a large cone of light is collected; and (2) the ability to change 

 the image size on the film. However, the field is restricted to the diameter of the 

 condensing lens. This method is useful for taking high-speed motion pictures on 

 continuously moving film with stroboscopic light. 



The last two arrangements of Fig. 1 illustrate methods of the "Schlieren" type of 

 silhouette photography. The upper diagram shows the use of a lens, and the lower, of 

 a mirror. In both most of the light is cut off by stops, and variation of the refractive 

 index of the material in the field of view deflects the light rays passing through the 

 affected region slightly, so that they clear or strike the stop and thereby either increase 



