SOME SPECIAL PEOCESSES 271 



should be illuminated by reflecting the light on to their surface 

 (Fig. 75) ; whereas with those having transparent colonies 

 (Fig. 76) the beam is projected from behind them, but at an 

 oblique angle so that the colonies themselves show up light on 

 a dark background. 



Fig. 77 a, b, c, d, e and / are examples of tube-cultivations. 



a. Coagulation of Milk by Bacillus coli communis. Photo- 

 graphed by reflected light, and with the illuminant placed well 

 below the level of the tube so that reflection from the tube 

 itself was thrown upwards. There is, however, some slight 

 evidence of tube-reflection, a streak showing just near the 

 centre, but it is hardly sufficient to interfere with the result. 

 The culture-tube was not immersed in any tank and shows 

 that it is possible to diminish tube-reflections greatly by suitable 

 adjustment of the light. 



b. B. coli communis, Gelatin Shake-culture showing gas-pro- 

 duction. Photographed by directly transmitted light. Instead 

 of the opaque screen the ground-glass screen was interposed 

 between the source of light and the tube. 



c. Klebs-Loffler or Diphtheria Bacillus on blood serum. 

 Oblique culture with surface colonies, showing very little 

 differentiation so far as the colour is concerned. The colonies, 

 however, have a glistening highly reflecting surface, and by 

 careful arrangement of the light it is possible to bring them out 

 showing brightly on the less bright ground. No water -bath was 

 used in this case, but the result was obtained by reflecting the 

 light from a point well above the level of the tube. 



d. Bacillus tuberculosis on agar-agar. Photograph obtained 

 by immersing tube in a water-glycerin bath ; illumination, by 

 reflected light. 



e. Bacillus of Tetanus, Stab-culture in sugar-agar. A rather 

 dense agar-tube with growth entirely in depth and at centre 

 of tube. A white screen was placed in the position usually 

 occupied by the opaque screen, and light was reflected from 

 the mirror on to the surface of the tube. The illumination 

 was well above the level of the tube, so that reflections were 

 thrown downwards. 



/. Bacillus tuberculosis on agar-agar. Very oblique illu- 

 mination of a surface-growth on agar. The light was placed 

 at a very acute angle to the surface of the agar in the tube, so 



