320 



HISTORY OF CHRONOPHOTOGRAPHY. 



at the same time to give them precision. The principle of the iirst 

 method employed was as follows: 



Suppose an ordinary camera to be pointed at a perfectly dark field, 

 and that an opaque disk in front of the lens is pierced with narrow 

 open i nos and turns about its center. Every time an opening passes 

 before the objective the light would be admitted, if there were any 

 light in the field. But there being no light, none penetrates the 

 camera; and when the plate is developed it is seen not to have been 

 affected. If a strongly lighted man or animal were to cross the dark 

 Held, each admission of light would produce an image of the animal, 

 and as the latter moved, photographs of it would be taken on the plate 

 at different places and in different attitudes. Such an arrangement, 

 however, would not answer. Fig. 4 shows the apparatus in its real 

 form. Within a cubical box is seen the camera with its lens. Behind, 

 it is the plate holder or back, C, which slides in grooves. Between 



the plate holder and the 

 r c : camera revolves the slitted 



disk grazing the sensitive 

 plate — in short, what is called 

 a plate-shutter. This disk, 

 I), with its narrow openings, 

 f . is worked by a clock move- 

 ment furnished with a speed 

 governor, and is set in mo- 

 tion by a handle. Fig. 5 

 (PI. I) shows the flight of a 

 white duck, which passes 

 before the dead black back- 

 ground. The succession of 

 images is from left to right. 

 Eight different attitudes are shown during one complete stroke of the 

 wings. They reveal the details of the mechanism of flight. In order 

 to appreciate the dimensions of the animal and the extent of its flight, 

 a divided rule is placed before the dark field. It is photographed and 

 serves as a scale. Finally, in order to show the intervals of time 

 between the successive images, at the lower right-hand corner of the 

 dark field is placed a chronograph, consisting of a dial, which has a 

 white hand completing an entire revolution in a second. Every time 

 the shutter disk admits light and causes a photograph of the bird this 

 hand is likewise photographed. Since it is seen to occupy eight suc- 

 cessive equidistant positions on the dial, it is evident that the intervals 

 have all been one-eighth of a second. 



\". .' h Dark field for chronojphotograpTiy on a fixed plate. — No body 

 is quite black. Chevreul showed that absolute blackness can only be 

 procured by means of a hole into a cavity with blackened walls, upon 

 which no light is allowed to shine. [That is. there should be another 



