1042 
(e) 
The resulting picture is reproduced in Figure 130. The image of the shock wave 
sphere and the image of the shadow projection of the shock wave sphere are seen 
nearly to coincide. On the average, however, the radius of the shadow projection 
appears greater than the radius of the shock wave by an amount corresponding to 
0.25 in. The geometry of the experiment would require a radius difference of 
0.65 in. The discrepancy is within the limits of experimental error. 
Luminosity =: charges exploded underwater. -- It is shown elsewhere in this report 
Sec. III, 2) that bare charges of cast pentolite were observed to emit light when 
detonated underwater. Photographs indicate this light to be of very short duration, 
probably less thanl ésec, because of the detail visible (numbers on the charges, 
cracks, etc.) and because there is no blurring. Foreseeing possible uses for this 
flash of light (e.g., accurately signalling at a distant point by means of a photo- 
electric cel? the time at which a charge detonates), several tests were conducted 
to determine if a cased charge would emit light on detonating underwater. 
In all tests, the charge, weighing from 1/2 to 1 lb., was placed 5 ft. in front of 
the camera and the lens aperture was set at f:3.5. A short piece of primacord, 
to be detonated simultaneously with the charge, was also included in the field of 
view to serve as a standard for comparison, since primacord had been found to emit 
light on detonating underwater (See Figure 104). All tests were conducted at night. 
The results showed that a heavy opaque casing reduced the emitted light to the 
extent that it was no longer detectable. For example, a charge wrapped with black 
rubber tape and detonated barely showed on the film, and a charge enclosed in a 
piece of steel pipe with pipe caps on the ends resulted in no image at all. Less 
heavy casings, however, such as 1/16 in, thick brass tubing, tin cans, and black 
lacquer resulted in weak but definite images of the charge and sometimes streamers 
of light emanating from the charge. There was some indication that aluminized 
explosives (torpex and minol) emitted more light than tetryl or pentolite under 
the same conditions, but the evidence is insufficient on this point. 
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