PHOTOGRAPHY OF UNDERWATER EXPLOSIONS 223 



traverse, and hence cannot be explored in detail. In many cases this 

 objection is not serious, however, and the exceedingly simple equipment 

 needed for optical distortion studies recommends the general method as 

 a potentially very useful one. 



C. Other refractioji methods. Although measurements of optical dis- 

 placements are the most promising means of utilizing refraction effects 

 in pressure waves having discontinuous fronts, two other possibilities 

 should be mentioned. The first of these is an interferometer method 

 devised by E. Mach and further developed by his son L. Mach (70), 

 which is essentially an application of the principle of the Rayleigh or 

 Jamin interferometer. The experimental arrangement consists in split- 

 ting light from a slit source into two geometrically separated beams by 

 use of a half-silvered mirror. These two beams are recombined at a 

 second half-silvered mirror after reflection from suitably placed re- 

 flecting surfaces, and interference fringes are formed in the combined 

 beam. If the refractive index of a finite region in one beam is changed, 

 the corresponding part of the fringe system is displaced in proportion 

 to the total change in optical path in this part of the beam. The method 

 is of course very sensitive to small changes in refractive index and has 

 been extensively used for investigation of supersonic flow in air or gases. 

 The differences in refractive index for liquids are so much greater, how- 

 ever, that even moderate pressures would cause impractical magnitudes 

 of fringe shift. Deviations in the path by refraction would make ele- 

 mentary analysis based on undeviated rays inapplicable. The most 

 useful application of interferometer methods is to small, slowly chang- 

 ing velocity gradients which are not characteristic of the more interest- 

 ing underwater explosion phenomena. 



A more promising possibility is the Schlieren method, in w^hich the 

 region of disturbance causes angular deviations of parallel light from a 

 source bounded by a straight edge. The beam is brought to a focus by 

 a lens in whose focal plane a second straight edge parallel to the first is 

 placed to block off all but a narrow strip of light. The deviations in the 

 beam result in more or less light passing this barrier into a camera 

 focused on a suitable plane in the disturbance. The original intensity 

 of the disturbance as recorded by the camera will then be changed in 

 proportion to the angular deviation. The method is less sensitive to 

 changes in refractive index than are interferometer techniques, but the 

 limitation of infinitesimal deviation of the beam must be respected if 

 elementary analysis is to be applied, and large discontinuities or gradi- 

 ents are handled with increasing difficulty. The method is neverthe- 

 less a potentially useful one, but has not to the writer's knowledge been 

 applied to studies of pressure waves in water. ^ 



^ The mathematical developments and limitations of this and other refraction 

 methods have been discussed in a report by Weyl (119). 



