244 PHOTOMICROGRAPHY 



slide is turned over on a sheet of blotting-paper and a 

 slight even pressure applied, but yeast cells are very 

 easily ruptured by a more localised pressure such as may 

 be given by accidentally bringing the objective down on 

 the cover-slip. It is necessary to apply a certain amount 

 of pressure in order to get as thin a film as possible, 

 otherwise it may be difficult to find a field in which the 

 organisms all lie in one plane. The mount should be laid 

 aside for some time to allow the cells to settle down on 

 the slip, and it is as well to ring the cover-glass with 

 melted paraffin wax or thick oil to prevent evaporation 

 of the liquid and consequent movement in the preparation 

 during examination and exposure. Every care must be 

 taken to prevent such movement among the organisms, 

 and a vertical camera is much the most suitable for this 

 reason. Bacteria may remain in motion, vibratory or 

 otherwise, for a long time, and the only possible way to get 

 a sharp picture is by means of a shutter with speeds of T V 

 to itW of a second. A very intense light is needed for such 

 exposures with high magnifications ; a 4-ampere arc or lime- 

 light is suitable, and the use of a cooling-trough is ad- 

 visable as the preparations are very susceptible to heat. 

 In order to obtain sufficient contrast between the 

 organism and background the iris of the substage con- 

 denser is closed until only about half of the aperture of 

 the objective is utilised. Extreme care must be exercised 

 in manipulating the diaphragm, as it is very difficult to 

 get the necessary contrast without producing a white 

 diffraction ring round each cell. The diffraction effect 

 may make itself very apparent in the photograph even 

 when very difficult to detect during the preliminary 

 examination of the object and adjustment of the iris. 

 A green screen may be used with advantage, but it must 

 often be dispensed with in order to allow of a sufficiently 

 short exposure, and the best results are obtained on 

 panchromatic plates. 



