CH.VII] PHOTOGRAPHY WITH DARK-GROUND ILLUMINATION 213 



§ 336. Photographing bacterial cultures. — For the successful 

 photographing of these cultures dark-ground illumination is employed 

 on the principle stated in § 117. That is, the preparation is illumi- 

 nated with rays so oblique that none can enter the objective directly. 

 These striking the culture are reflected into the objective. The clear 

 gelatin around the growth or colonies does not reflect the light and 

 therefore the space between the colonies is dark. 



For supporting the Petri dishes a hole is made in a front board for 

 the camera. This hole is sHghtly larger than the dish. Over it is 

 then screwed or nailed a rubber ring slightly smaller than the Petri 

 dish. This will stretch and receive the dish, and grasp it firmly, "so 

 that it is in no danger of falling out when put in a vertical position. 

 If the camera has two divisions like the one shown, the board with 

 the Petri dish is put in the front of the camera, and the objective in 

 tlie middle division through the side door. Otherwise the board 

 holding the Petri dish must be on a separate support (fig. 108). 



The vertical camera and focusing stand (fig. 126) lend themselves 

 admirably for this kind of photography. The black background can 

 be put on the lower stage and the Petri dish or other bacterial culture 

 can be set on a glass plate or in a perforated board on the upper 

 stage. The hghting is very easily accomphshed by two portable 

 lamps so arranged that no hght can get directly from them into the 

 objective. 



One may use daylight by putting the culture in a support just out- 

 side a window, leaving the camera in the room. The rays from the 

 sky are so oblique that they do not enter the objective. One must 

 use a black non-reflecting background some distance beyond the dish 

 as in using artificial hght (Atkinson) . 



In photographing bacterial cultures in test-tubes the hghting is 

 as in the preceding section, but a great difficulty is found in getting 

 good results from the refraction and reflections of the curved surfaces. 

 To ovecome this one applies' the principles discussed in § 202, and 

 the test-tubes are immersed in a bath of water or water and glycerin. 

 The bath must have plane surfaces. Behind it is the black velvet 

 screen, and the light is in front, as for the Petri dishes. As suggested 

 by Spitta, it is well to employ a bath sufficiently thick in order that 



