82 MOVEMENT 



second exactly from the side, and in the third from 

 behind. These differences in perspective are less 

 appreciable if the apparatus is removed further off, 

 and the respective images become more easy to com- 

 pare and to measure. But by moving the camera 

 further away the images become smaller, and hence it 

 is necessary to use an objective of greater focal length 

 in order to obtain large enough images. We need 

 say no more on this subject, since text-books on 

 photography give most exact directions. 



Focussing. — The object is brought into focus on 

 the ground-glass plate in the slide (Fig. 47). The 

 apertures of the two diaphragms must be made to 

 coincide by turning their axes with the hand, and 

 the image is seen through an opening situated at the 

 back of the apparatus above the crank.* 



How to take the Photograph. — Just as the aperture 

 of the box is kept within strictly necessary limits, so 

 too is the length of the exposure reduced as much as 

 possible. If the sensitized plate be unnecessarily 

 exposed before or after the end of the phenomenon, 

 the intermittent exposure of the objective will allow 

 access to the plate of small quantities of light, which 

 tend to cause "fogging." This inconvenience can be 

 avoided by placing in front of the objective a special 

 diaphragm, of the kind which is worked by pressing 

 an indiarubber ball with the hand. This anterior 

 diaphragm, when shut, makes it possible to open the 

 shutter of the dark slide, to place the apparatus in 

 order, and prepare for the experiment, the sensitized 

 plate being meanwhile in darkness. 



At the moment the phenomenon commences, the 



* This portion of the posterior part of the apparatus contains a 

 special chamber adapted for photographing upon a moving film, which 

 will be mentioned later. It is through this chamber that the image 

 can be seen upon the ground-glass plate. 



