PHOTOGRAPHIC METHODS 



111 



This iron tube, or camera tube, is lowered and raised 

 in the well by a cable winding of the lower of two drums e 

 and / (Fig. 15). The upper drum / carries an electric 

 cable to operate the lamps and to turn the camera film. 



The cable is so fastened to the tube that the window 

 will come close to the wall of the well, and, with the lights 



Fig. 116. — Atwood's borehole camera with parts removed to their relative places 



outside the case. 



burning, the wall is brightly illuminated. In making an 

 exposure with a No. 16 stop the lights are turned on for 

 about 20 sec. Before making a second exposure the 

 camera is lowered or raised 43-^ in., the 

 distance covered by one photograph, 

 and a new part of the film is turned 

 into place by making and breaking the 

 circuit of an electromagnet acting upon 

 the roll of film. In this way a series of 

 50 or more photographs can be taken at 

 the rate of 1 a minute, and they will 

 show a continuous strip of the wall for 

 a distance of 20 ft. or more. The win- 

 dow, which is 13^^ by 5K in. is set in 

 litharge cement. A guard strip is riv- 

 eted to the tube on each side of the 

 window. The hoisting cable is attached 

 to the hook g, 4 in. behind the window, 

 so that in the ordinary 6-in. drill hole the 

 window always hangs near the wall. The 

 mirror, lamps and reflectors are mounted ^^g- ii7.— Camera 



. , T , 1 y^^th. side removed. 



on an oak plate, which can be adjusted 



to bring the mirror in the right position behind the window 



(Fig. 117). The two lamps are 10 volts and 5 cp. each. 



