268 PKACTICAL PHOTO-MICKOGKAPHY 



fThe only point that requires special consideration in the 

 photographing of such objects is the means of lighting. In 

 general, tube- or plate-cultures of bacteria may be regarded 

 as either opaque or transparent objects, so that illumination 

 would be conducted as in microscopic objects, either by trans- 

 mitted or by reflected light. As a compromise a method of 

 oblique lighting is often most useful, and where small semi- 

 transparent colonies are lying on a gelatin or agar surface it 

 is generally the only way to obtain any differentiation between 

 the object and the medium. 



It is also to be remembered that the two methods of growing 

 bacteria in tubes, usually referred to as stab or streak cultures 

 in which the growth is either in the depth of the medium or 

 on its surface, will require illuminating in different ways. The 

 point to guard against particularly is that, while the surface- 

 growth or the growth in depth has to be properly illuminated, 

 it must b'e done without any light being reflected from the 

 internal or external walls of the glass tube itself, for such 

 reflections interfere seriously with the proper delineation of 

 the object. One method of avoiding reflections from the 

 tube is to immerse it in a rectangular glass vessel which is 

 filled with water, or water and glycerine. With gelatin- 

 cultures, water will usually be found sufficient, as its refractive 

 index sufficiently nearly approximates to that of the gelatin ; 

 but with agar or more highly refractive media, then the water 

 and glycerin combination is better. The exact proportions 

 of the mixture are best determined by observing the effect on 

 the appearance of the tube when more or less glycerin is added 

 to the water ; but in general two parts of water to one part of 

 glycerin will be found suitable. With the tubes placed in a 

 glass cell in this manner, it is only necessary to throw an 

 oblique beam from a fairly powerful source of light from a point 

 either in front or behind the tube. The camera is placed in a 

 suitable position in front of the culture, and is so extended that 

 it gives an image which is approximately the same size as the 

 culture-tube itself. The lens will then require to be stopped 

 down to No. F 11 or F 16 to obtain sufficient depth of focus. 

 It may be necessary in order to obtain the best illumination 

 to vary the angle of incidence of the light in a vertical direction 

 so that the light is projected from above the point of view 



