140 PRACTICAL PHOTO-MICROGRAPHY 



about such an arrangement is that it should be possible to 

 observe the microscopic image, and to focus up that image until 

 the desired appearance is seen ; then to be able to take an 

 instantaneous photograph at the exact moment when this 

 appearance is observed and the object is in focus. A simple 

 way of doing this is to utilise an ordinary reflex photographic 

 camera (Fig. 52). In these cameras, it is well known that when 

 they are used in the manner and for the purpose for which 

 they are designed, an object can be seen and focussed right up 

 to the moment of exposure. For photo-micrographic work one 

 of these may be used in conjunction with a vertical camera of 

 the Van Heurck type already described. It is obvious that a 

 camera of the vertical type is the one most useful for this class of 

 work ; for it almost invariably happens that living microscopic 

 objects are immersed in fluid, and that therefore it is essential 

 for the microscope to remain in a vertical position. The 

 ordinary ground-glass focussing- screen at the upper end of 

 the Van Heurck camera should be removed, and in its place a 

 wooden frame to carry the reflex camera is fixed. This frame 

 should have an opening in its centre say, of three inches 

 diameter which is sufficiently large for any microscopic object 

 to be projected through it. The photographic lens on the 

 reflex camera, and if possible the whole of the front of 

 the camera carrying the lens, should be removed, and the 

 camera turned over in the position as shown, so that it 

 lies in a horizontal position, and allows the fine-adjustment of 

 the microscope to be reached while observing through the 

 focussing-hood the image on the ground-glass screen of the 

 camera. The only limit to the employment of this method, 

 so far as higher powers are concerned, is the intensity of the 

 illuminant that is in use. It is obvious that with any dark- 

 ground illumination method, to obtain an instantaneous 

 photograph the light-source would require to be powerful, and 

 it is very doubtful whether anything less than an electric arc 

 would be of service. In any case the camera would require 

 to be used in conjunction with one of the high-power illumin- 

 ating methods already described. It is essential that the reflex 

 camera utilised for this purpose, although it may be of very 

 simple design, should be such that it does not cause any 

 appreciable amount of vibration on releasing the reflecting 



