666 Subsurface Geologic Methods 



that a sample of it in a pint milk bottle completely obscured any vision 

 through the bottle. It is, however, always an advantage to have the well 

 fluid as clear as possible. This is particularly true where there may be 

 difficulty in having the camera fit up tight against the formation or where 

 the well wall may be rough and irregular. 



The diameter of the water chamber is, of course, limited by the dia- 

 meter of the smallest well in which the apparatus is to be used. Making 

 the water chamber large is an advantage, because this provides a large 

 picture window and increases the area of the well wall that is photo- 

 graphed. Furthermore, the greater the diameter of the cylindrical picture 

 window the better it will fit up against the rock, thereby improving the 

 pictures if the well fluid is turbid. Making the water chamber fairly long 

 is also an advantage, as this increases the distance from the lens to the 

 subject and the depth of field. This facilitates sharp focusing even when 

 the well wall is irregular with various parts of the formation at different 

 distances from the picture window. 



The camera requirements are not severe. The field angle of the cam- 

 era should be sufficiently wide that the extent of the subject covered is 

 limited only by the mirror or picture window. Owing to refraction at 

 the pressure window the subject appears to the camera to be only three- 

 fourths as far from the camera as it actually is. The lens is focused ac- 

 cordingly and permanently locked. To increase the depth of field, the lens 

 may be stopped down to f/16 or smaller. No shutter is needed, as ex- 

 posures can be made by turning on the light for the desired exposure 

 time and turning it off while forwarding the film and shifting the camera 

 to the next location. The camera must have enough film capacity that 

 frequent reloading is not necessary, and the film-drive mechanism must 

 be controlled electrically. 



The light or lights must be located so as to give satisfactory illumina- 

 tion of the subject without undesirable reflections from the window and 

 mirror, must be shielded from the camera lens, and must be able to stand 

 the well pressure. 



The Equipment 



Figure 355 shows the actual construction of one of these deep-well 

 cameras. The glass cylinder forming part of the water chamber is four 

 inches in outside diameter by 4^ inches long by one-fourth inch thick. It is 

 surrounded by a steel sleeve having a rectangular opening about three 

 inches wide by 4i inches high in one side, forming the picture window. 

 The ground ends of the glass are sealed by means of "neoprene" gaskets 

 against the steel head and steel nosepiece, which are screwed into the 

 steel sleeve. These joints are subjected to only small differential pressures 

 because of the equalizing action of the bellows in the nosepiece. Attached 

 to the head is the glass mirror making an angle with the vertical some- 

 what less than 45°. The mirror is shaped elliptically to fit inside the 



