Miscellaneous Subsurface Methods 667 



glass cylinder. The back surface is silvered. In the top of the head is the 

 glass pressure window sealed with a thin rubber gasket to withstand the 

 high pressure in the water chamber. This window is half an inch thick by 

 1^ inches in diameter with an unsupported diameter of three-fourths inch. 



The head carries two General Electric No. 1129 21-candle-power auto- 

 motive-type lamps, which are operated on six-volt, sixty-cycle alternating 

 current. These lamps were found to withstand the high external pressures 

 satisfactorily. Using distilled water in the chamber with lamp contacts 

 and connections insulated from the water, the leakage current was found to 

 be negligible, and no bubbles due to electrolytic decomposition of the 

 water could be observed. The exact position of the lights and the shape 

 of the reflectors and shields were chosen to give good illumination of the 

 subject while avoiding reflections in the mirror and glass cylinder, which 

 might otherwise show up as bright spots in the picture. Tipping the mirror 

 slightly steeper than 45° also helped to eliminate these reflections. 



The camera is screwed to the top of the water-chamber head with its 

 lens directly above the pressure window. The camera housing slides down 

 over the camera making a pressure-tight threaded connection with the 

 watertight chamber head. This joint is the one that is taken apart whenever 

 the camera is loaded or unloaded. The camera housing, which is 4^^ inches 

 outside diameter with a three-sixteenth-inch wall, was tested with an ex- 

 ternal pressure of 2,500 pounds to the square inch. The camera housing 

 and water chamber are zinc-plated inside and out for corrosion resistance. 



The Camera 



The camera itself, shown in figure 356, is a modified Bell and Howell 

 model 151 magazine-loading 16-millimeter movie camera with a 7.4-milli- 

 meter by 10.3-millimeter frame size. It has a built-in feature permitting it 

 to take single exposures in the same manner as with any still camera. The 

 control button has been coupled to a solenoid for remote electrical control. 

 The power for driving the film still comes from the spring motor of 

 the camera, which will give 450 exposures on one winding. A lens having 

 15-millimeter focal length is used, and provision has been made for 

 stopping the lens down as small as f/32 if desired for greater depth of 

 field. At f/32 an exposure time of eight seconds is satisfactory when neg- 

 ative panchromatic safety film is used. 



The upper end of the camera housing is threaded to make a water- 

 tight connection to the socket on the end of the electric cable. Below this 

 connection is a "fishing stub" to facilitate retrieving the camera from a 

 well in case of accident. A transformer fixed in the upper end of the 

 housing provides the six volts required by the lights. When the housing is 

 in place, two spring contacts below the transformer engage contacts on the 

 upper end of the camera to provide connections to the camera and lights. 



Two springs (see fig. 355) with their upper ends fixed to the camera 

 housing and their lower ends attached to a sliding block hold the camera 



