56 MOVEMENT 



of the object in the various phases of motion. The 

 interval between each image was exactly ^ of a second, 

 and the duration of the exposure 5 ^$ of a second. A 

 metre rule with very clearly defined divisions was 

 placed in front of the screen, and in the same plane 

 as the object. The image of this rule reproduced on 

 the sensitized plate served as a scale to measure the 

 real size of the object, and the spaces traversed during 

 each period of j 1 ^ of a second. 



Instead of depending on the absolute regularity 

 of the movement of the diaphragm as a means of 

 measuring the time relations, it would be better in 

 experiments requiring great accuracy to make use of 

 the chronometric dial (Chap. I., Fig. 12). Thus in the 

 experiments on falling bodies described on page 51, 

 the intervals of time between two successive exposures 

 were measured by the angular distance through which 

 the needle moved between two successive images. 

 This proceeding, like that in which the tuning fork is 

 employed as a mechanical means of registering the 

 rate of movement of the paper, permits of the diaphragm 

 revolving at any speed required. The degree of speed 

 can always be ascertained by referring to the position 

 of the needle on the dial. 



As for the measurement of space, the image of a 

 divided scale serves, as we said, for measuring the 

 various distances on the photographic plate. 



But, since all measurements made from a reduced 

 scale necessitate a series of calculations before the 

 real dimensions are ascertained, it is very desirable to 

 find some means of avoiding these tedious calculations. 

 This may be done by enlarging the images, by means 

 of a projection lantern, until the object assumes its 

 actual dimensions, i.e. until the measuring scale on the 

 screen appears to be exactly one metre in length. In 

 this case all the dimensions of the image can be directly 



