INSTANTANEOUS PHOTOGRAPHY. *31 



wide (approximately thirteen feet by ten feet), covered, some with 

 black, others with white, cloth, over which were stretched threads 

 contrastingly white or black, forming squares exactly like those 

 in the lateral background at the studio. They could be set up 

 and levelled in a few moments back of the course over which the 

 animal or bird was to pass, and were firmly held in position by 

 guys. 



The background being arranged at the studio or away from it 

 in the manner described for each, it will be readily understood 

 that every photograph in a multiple series had upon it, behind 

 the figure, a number of equidistant parallel lines, both horizontal 

 and vertical in all except the ninety-degree foreshortenings made 

 at the studio, where they were vertical only. Now, by noting 

 the positions of any part of the body upon the background of 

 squares in the consecutive photographs of a lateral series the 

 amount of forward and upward motion between successive phases 

 can be accurately determined ; and in the same way, by means of 

 the background in the ninety-degree foreshortenings, the amount 

 of a sidewise movement can be found. From these determina- 

 tions the curves which different parts of the body describe may 

 be readily plotted. These trajectories, or those described by the 

 same limbs of different animals in performing the same move- 

 ments and so forth, can be critically compared, and at the same 

 time the corresponding amount of muscular action shown in the 

 photographs can be examined, with results of probably greater 

 interest and value than we can at present foretell. 



During the investigations in the summer of 1885 a very inter- 

 esting double series of photographs was made to determine the 

 equality of the intervals between successive exposures, the accu- 

 racy of the chronographic record of these intervals, and the dura- 

 tion of the shortest exposures used in the work, Mr. Muybridge's 

 statement that in some cases the exposure lasted but one five-thou- 

 sandth of a second having been doubted by several critics. Short 

 photographic exposures are usually measured by photographing 

 some object moving rapidly at a known or readily-determined 

 speed. The blur in the photograph shows how far the object 

 moved while the picture was being taken, and from these data of 

 distance and speed the duration of the exposure is calculated. The 

 usual objects photographed (a falling ball, a swinging second pen- 



