246 PHOTOMICROGRAPHY 



obliquity of illumination, produced by decentring the 

 substage condenser to a very small extent, gives the solid 

 image which more nearly reproduces the contour of the cells 

 than the flat image obtained by exact central illumination. 



Culture Tubes and Plate Cultures (Plates 37 and 4). 

 A difficulty arises in the photography of culture tubes 

 from reflections on the tube, which give white lines in 

 the print and quite obliterate the detail of the growths. 

 This is easily overcome by immersion of the tube in a 

 trough of water, alcohol, or glycerine, as shown in the 

 Plate. The reflections are there noticeable on the portions 

 of the tubes above the water. Flat-sided light filter or 

 cooling troughs can be used for this purpose, or cleaned 

 photographic plates may be cemented together with an 

 aquarium cement, such as that given on page 276. 

 Condensed moisture on the inside of the tubes should, 

 whenever practicable, be removed by means of a roll of 

 filter paper. 



The structure of most growths is best shown up by 

 illumination from the front, but in some cases illumina- 

 tion from behind gives a better result. The simple 

 apparatus of Fig. 40 is suitable in either case, the tubes 

 being placed close to the background and illuminated 

 from one or both of the lamps according to the shadows 

 required, or brought in front of the lamp reflectors for 

 transmitted light reflected from the white background. 

 If a more intense illumination is required the principle 

 remains the same, but condensers are used to throw light 

 from one or more lamps in front of or behind the tubes, 

 at whatever angle gives the best relief and shows the 

 structure to the greatest advantage. The lens is shaded 

 from direct light from the lamp. Diffused light, daylight 

 or artificial, may be more convenient than the above ; 

 an electric arc without a condenser was used for Plate 

 37. White or black backgrounds are selected according 

 to the nature of the object. 



