272 PEACTICAL PHOTO-MICKOGBAPHY 



that the greatest possible contrast was obtained to show up 

 the convolutions in the growth itself. 



The exposure required in such cases must of course be 

 dependent on the illuminant ; but using a source of light 

 such as a one-ampere Nernst lamp under the conditions 

 described, none of the exposures for the examples here given 

 exceeded three minutes. A plate of medium rapidity should 

 be used, so that the greatest possible contrast is obtained. 



It should be remembered, when usi^g any source of light of 

 high power, that gelatin-cultures particularly will melt down 

 with a relatively small rise of temperature ; hence the tubes 

 should be well shielded from any direct light, and if they are 

 contained in a water-bath should not be left long enough for 

 any rise of temperature of the water to take place. In summer 

 even a rise of four or five degrees will be quite sufficient to 

 melt the contents of the tube. 



It is also important to remember that if the tubes are 

 contained in a water-bath, the bath itself as well as the 

 outside of the tube should be perfectly clean, as any impurities 

 will show up only too clearly in the photographs. 



A condition that sometimes does arise and gives a great 

 deal of trouble is that due to the condensation of water-vapour 

 on the inside of the culture-tube itself. In cases where 

 preparations are permanently sealed the difficulty is a very 

 considerable one. If such condensation appears, it is generally 

 due to the tube being taken for photographing into a room, 

 the temperature of which is lower than that of the incubator in 

 which the growth has been allowed to develop. In certain 

 cases it may be possible to remove the wool plug and by means 

 of a small bit of cotton-wool held in long forceps to wipe the 

 condensed vapour off the inside of the tube. This must be done 

 with the utmost care to avoid disturbance of surface-growth, 

 and in dealing with pathogenic organisms the usual laboratory 

 precautions should be taken. A better way, in cases where 

 the cultures cannot be interfered with, is to place the tube for a 

 short time in a box containing some calcium chloride. In the 

 course of from two to three hours this will usually absorb suffi- 

 cient moisture to cause evaporation of any free water -vapour 

 in the tube which might otherwise be sufficient to prevent a 

 photograph being taken. 



