LOW-POWEK WORK 27 



nutrient medium stiffened with agar or gelatin. They commonly present two varieties, 

 viz., " stabbed " and " streaked " cultures. In the former the medium is solidified whilst 

 the tube is held in a vertical position ; in the latter when it is almost horizontal. In 

 stab-cultures the growth is seen down the centre of the medium, in streaked cultures 

 along its inclined surface. The difficulty in photographing them lies in the fact that 



Fig. 1 6 

 Elevation 



the glass walls of the tube reflect so much light as to spoil the final appearance of 

 the picture. To avoid this it is best to place them in a large water-bath filled either 

 with water or water and glycerine, the object of so doing being to entirely, or anyhow 

 very largely, do away with reflections of all kinds. The lens we have found most 

 suitable is a Ross Goertz or an anastigmat of about 5 inch focus, using the ordinary 

 \ plate camera. 



The arrangement as previously described (Fig. 13) should now be placed on the 

 extra slide with rails (that we suggested should be made), as it enables the 

 photographer to obtain his picture with comfort, for the camera slides to and 

 fro with ease and axial precision. The use of the levelling screws mentioned is now 

 apparent, for by them the photographer can adjust small irregularities in the 

 verticality of the tube without the necessity of touching it in its bath. The position 

 of the lime and condenser are shown in the diagram. Fig. 15. The exposure, when 



