CRITICAL PHOTOMICROGRAPHY 143 



Vertical Cameras. The instructions given in this 

 chapter and the accompanying illustrations refer par- 

 ticularly to horizontal cameras, but they apply equally 

 well to vertical cameras, and can in some respects be 

 carried out more easily under those conditions. The 

 adjustments are just the same, except that the direction 

 of the beam of light is changed by the mirror. The 

 microscope is placed under the camera, and centred by 

 means of the disc of light projected on the ground glass. 

 Centration of the light is effected very easily by move- 

 ment of the mirror, while the condensers, light-filters, 

 cooling trough, etc., are disposed exactly as for a hori- 

 zontal camera. If the mirror can be arranged on the 

 level of the condensers and lamp, their optical axis is kept 

 horizontal, otherwise the condensers are tilted down, as 

 shown in Fig. 7. Light-tight connection between the 

 camera and microscope is made either by the double 

 cells or velvet sleeve (Figs. 4 and 71). 



Preliminary Examination of the Object. Before pro- 

 ceeding to take a photomicrograph it is necessary to 

 make oneself familiar with the object and its character- 

 istics, and decide what details, if any, are to be particu- 

 larly brought out, and under what magnification or 

 method of illumination it is best displayed. The examina- 

 tion is usually carried out in the following way, after a 

 careful centra tion of the condenser, etc. 



1. Search the whole preparation with a low-power, and 

 decide whether the photomicrograph is to include the 

 whole or only part of the object, and find the positions 

 that best display the particular points of importance. 



2. Locate the desired points, if necessary, by means of 

 the verniers fitted to the mechanical stage, so that they 

 may be rapidly found when using a high-power at any 

 future time. 



3. Examine the object with various combinations of 

 objective and ocular, and decide which combination is 



