242 PHOTO-MICROGRAPHY [ CH. VIII 



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 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 verti- 

 cal 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 on 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 objective. 

 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 win- 

 dow, 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 light (Atkinson). 



\ 392. Photographing Bacterial Cultures in Test-Tubes. — Here the lighting 

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

 sults from the refraction and reflections of the curved surfaces. To overcome this 

 one applies the principles discussed in $ 144, 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 sloping surface will all be in focus 

 at once by inclining the test-tube. 



See the works on photo-micrography and photography for the details of lan- 

 tern slide making. See for the Petri dishes and test-tubes, Atkinson, Botanical 

 Gazette, xviii (1893), p. 333 ; Spitta, Photo-Micrography (1899), p. 26. 



