764 



HANDBOOK OF PHOTOGRAPHY 



or decrease the amount of light reaching the film at the corresponding image points. 

 In this way variations in density in a volume of gas, such as those produced bj^ sound 

 waves, and thermal disturbances, may be observed and photographed. 



An optically perfect lens or mirror is required for Schlieren photography in order to 

 obtain a uniformly illuminated field. In fact, the arrangement is exactly the same as 

 the test used for the final polishing of optical surfaces. 



The size of field is naturally limited to the size of the lens or mirror. Since cor- 

 rected lenses of large diameter are very expensive or not obtainable, mirrors are more 

 commonly used. The references show numerous different optical arrangements. 



Shadow photography is often of great advantage in many practical problems 

 because the amount of light required is small compared with that needed for reflected- 

 light photography. Furthermore, a sharp image is obtained which is useful for meas- 

 urement. The waves in air, such as are caused bjj- bullets, may be recorded by the 

 silhouette methods, especialh^ the Schlieren method, which can be made very sensitive. 

 In fact the Schlieren arrangement can be made sufficiently sensitive to observe heat 

 waves rising from one's hand, because of the fact that the index of refraction of air 



About 



—.^/^ I — X /Mercury 

 _ T\o/7ms a confrot 

 8,000V.^^j;^, tube 



Om'^^ Amplifier 



'^^.^It-^ irip 

 ^Trip switch 

 St robot ron or th^/ratrorr' 

 Fig. 2. — Electrical circuit for making silhouette photographs. 



changes with temperature. The principal disadvantages of the silhouette method are 

 that only an outline is obtained and that with some kinds of shadow photography the 

 experiment must be performed in a darkened room. 



The electrical circuit of a typical arrangement for taking silhouette photographs 

 is shown as Fig. 2. The various circuit elements are labeled in the figures. Many 

 other circuits are described in the references, which should be consulted for further 

 details. 



Since the light from the sparks in air and from most of the other gaseous-discharge 

 sources is largely in the blue end of the spectrum, the less sensitive films or papers — 

 such as the process or positive types — are often satisfactory. 



The duration of the flash which determines the exposure time for silhouette photo- 

 graphs depends upon the electrical arrangement and also upon the afterglow in the 

 gas. The duration is dependent on the natural frequency of the discharge circuit, 

 which in turn increases with the size of the condenser and the inductance resulting 

 from the connecting wares and the internal inductance of the condenser. Further- 

 more, discharges in circuits of this type usually oscillate a great many cycles — some- 

 times as many as 10 or more. For this reason sufficient resistance is often added to 

 damp critically the oscillations. This resistance decreases the duration of the flash 

 but at the same time reduces the quantity of light by from five- to tenfold, which is a 

 serious disadvantage. The mercury-control tube shown in Fig. 2 has the advantage 

 of tending to prevent the reversal of current, as well as serving as a switch for starting 

 the spark gap at the required instant. The ignition coil starts conduction in the mer- 



