CH. Vllf] PHOTO-MICROGRAPHY 243 



able for the purpose. For lantern slide work a photographic objective is used 

 and the cone for enlargement removed. One may put the objective in the 

 front of the camera or in the middle segment, making use of the little side 

 door. 



I 327. Photographing Bacterial Cultures in Petri Dishes.— For the suc- 

 cessful photographing of these cultures dark ground illumination is employed 

 on the principle stated in \ 103. That is the preparation is illuminated with 

 rays so oblique that none can enter the objective. These striking the culture 

 are reflected into the objective. The clear gelatin around the growth or col- 

 onies 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 slightly 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 the middle division through the side door. Otherwise the 

 board holding the Petri dish must be on a separate support. 



The illumination is accomplished by the use of two electric lamps with 

 ■conical shades. (The cheap tin shades with white enamel paint 011 the inside 

 are good). The lamps are placed at the sides so that a bright light is thrown 

 on the culture, but at such an angle that none of it enters the objective 

 directly. 



A piece of black velveteen is placed 10 to 20 cm. beyond the culture. This 

 prevents any light from being reflected through the clear gelatin to the objec- 

 tive. Unless some such precaution were taken the background would be gray 

 instead of black. 



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

 window, leaving the camera in the room. The rays from the sky are so 

 oblique that they do not enter the objective. 0:ie must use a black non-re- 

 flecting background some distance beyond the dish as in using artificial light 

 (Atkinson). 



I 328. Photographing Bacterial Cultures in Test-Tubes. — Here the 

 lighting 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 

 overcome this one applies the principles discussed in § 157, 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 streak cultures may be arranged so that the slop- 

 ing surface will all be in focus at once by inclining the test-tube. 



REFERENCES FOR CHAPTER VIII 



See the works and journals dealing with photography. 



For Photo-Micrography see Pringle, Bousfield, Neuhauss, 3rd ed. Stern- 



