MICROSCOPIC CHRONOPHOTOGRAPHY 301 



when it is required to observe the movements of certain 

 infusoria as they swarm about in the field of the 

 microscope. Hardly has the presence of these creatures 

 within the field of the microscope been assured before 

 they move out of range and necessitate a new adjustment 

 of the apparatus ; for instance, they will disappear 

 during the act of removing the prism which projects 

 the image upon the sensitized film, or while the 

 apparatus is being set in motion. Accurate focussing 

 is difficult to effect because the equalization of the 

 focus of the eye-piece and of the sensitized plate is a 

 delicate operation. 



These difficulties may be surmounted by the follow- 

 ing contrivance which seems to us to be applicable in 

 all cases. We locate ourselves in a dark room into 

 which the sunlight, reflected by a heliostat, can pene- 

 trate through a hole in the shutter. The luminous 

 rays are made to traverse a vessel containing a solution 

 of alum, and to converge on to the preparation by 

 means of a condenser. 



The image formed by the microscopic object-glass 

 is received upon a screen. A hole is made in this 

 screen of the same size and shape as the admission 

 aperture, and behind this hole the chronophotographic 

 apparatus is placed. 



The front part of the latter is removed and the 

 hinder part is provided with circular diaphragms 

 which rotate in front of the admission aperture. In 

 this way the images of moving creatures can be seen 

 crossing the screen, and the moment can be seized 

 at which these images appear at the aperture ; at this 

 moment it is clear that they must be within the 

 field of the sensitized plate which is situated exactly 

 behind. 



The knob may be pressed, and a series of photo- 

 graphs taken. If the image is noticed to leave the 

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