INSTANTANEOUS PHOTOGRAPHY. 27 



was rotated on its vertical axis by clock-work, and could be 

 moved up or down on the shaft by means of a supporting arm 

 working on a screw, O, so that the paper could be entirely covered 

 with records (often fifteen or twenty) before renewing. The drum 

 and shaft were then lifted out of the bearings, the paper cut off, 

 and the records fixed permanently upon it by immersion in dilute 

 shellac. The chronograph, its batteries, etc., were enclosed in a 

 wooden box, which at the studio stood on a shelf at C, Fig. 1. The 

 electro-magnet M, whose purpose has been already explained, was 

 operated by a shunt current from six cells of the battery C, the 

 circuit being closed by the contact-key A. The latter was de- 

 signed especially for portability, and consisted of two stiff brass 

 springs with platinum contacts, mounted on round sticks about 

 two feet long, so that it could be conveniently carried in the hand 

 from place to place; for the operator usually stood directly beside 

 the camera in which the first photograph of the series was to be 

 made, and by making the contact when the subject reached the 

 desired position the whole series of exposures was started. 



Having now described the apparatus in detail, we can, perhaps, 

 best understand the method of operation by imagining ourselves 

 in the studio at the University, watching the making of a series 

 of twelve photographs from each of three points of view simul- 

 taneously, thirty-six different negatives in all. In order to make 

 our observations more definite we will suppose that the action to 

 be investigated is one stride of a walking horse ; that the large 

 or " lateral" cameras, L, Fig. 1, are in their usual position parallel 

 to the line of motion on the track and at a distance of forty-nine 

 feet from it; that the portable cameras, E and F, have been 

 j)laced, one battery for front the other for " rear sixty-degree fore- 

 shortenings," at a distance of thirty-five feet from the centre of the 

 track ; that is, in places permanently marked so that they point in 

 a direction at nearly sixty degrees to that of the laterals on both 

 sides ; the lenses of all three batteries are in the same horizontal 

 plane with a certain heavy thread in the lateral background, the 

 " horizon line." 



The cameras being already focused, an assistant proceeds to set 

 the exposors, and the horse is started on a trial-trip down the 

 track that we may see about how much time he occupies in 

 making one s stride. We count ten strides in eleven seconds 



