28 PHOTOMICROGRAPHY 



employing an isochromatic medium plate by Edwards, well and evenly backed, is 

 about 30 seconds at F/32. 



When transmitted light is required the illuminant should be arranged as given 

 in Fig. 16, where elevation and plan are both shown. That is, it should shine 

 obliquely on the tube from above downwards. It is best so to arrange the light that 

 parallel rays issue from the condenser upon the water-bath, which is done by pushing 

 up the lime quite close to the condenser, in point of fact, to place it in its focus. 

 Approximately parallel rays will then issue from the lenses. The water-bath must 

 not be too small ; about 4 inches long, 2 inches wide, and 5 inches deep are convenient 

 dimensions. As a water-bath of this size is not a commercial article, but can be 

 readily made, the following instructions for its manufacture may be useful : Procure 

 two glasses of the correct size, some solid vulcanite (2 inches thick), cut in lengths to 

 form the bottom and sides. Cover each surface of the vulcanite that comes in 

 contact with the glass, as well as the surface of the glasses that come in contact with 

 the vulcanite, with Miller's cement— procurable at any optician s — and firmly press 

 the glasses on to them. Leave to dry for twenty-four hours. Now cut narrow strips of 

 good strong paper, and apply them over the points of union of the glass and the 

 vulcanite inside, and having coated them as well as the glass and vulcanite with 

 cement, again leave for twenty-four hours. After becoming dry, apply one or two 

 more coats over the joints with the same cement, and all will be complete. To make 

 it more handy and to prevent being easily upset, it is well to arrange a wooden 

 support to take the bath as a whole. 



With streaked cultures the tubes must be inclined at such an angle that the level 

 of the obliquely inclined medium is approximately at right angles to the optical axis 

 of the lens, by which means it will be found the upper and lower ends of the gelatin 

 inside the tube can be photographed at one plane of focus. It will now be seen why 

 it was necessary to make the water-bath so wide ; it was to allow of this tilting. 



Another variety of tube, which the bacteriologist may require to have photo- 

 graphed, has not yet been referred to, namely, the bubble-tube of Bacillus coli 

 co7nmimis, or that of B. oedematis maligni — malignant oedema. Such tubes contain 

 bubbles of gas which, arising in the substance of the medium, pass through it towards 

 the top. It is no easy matter to take a good photograph of this type of tube, unless the 

 light is placed almost in a line with the lens on the other side of the bath, a piece oi 

 ground glass being placed between the condenser and the water. The photographer is 

 reminded to be careful not to focus the ground glass — let him keep it near the condenser. 



As our desire is to make what we say essentially practical, a few hints must be 



