30 THE MECHANISM OF 



in a notch in the extremity of the armature-lever g, the latter 

 being held in place by a spring. The double arm d also catches 

 in g^ as explained before. Now, with g, /, and d thus arranged, 

 as shown in the diagram, let us suppose /, and with it s and o, to 

 be revolving at the desired speed. An electric current is sent 

 for an instant through the coils of the electro-magnet m. Its 

 consequent magnetic attraction pulls down the armature c, and 

 with it the lever g. The spindle s, no longer opposed by I, is 

 pushed outward by the force of the spring ; o is thus thrown into 

 gear with n, and the contact-brush on the arm d, now free to turn, 

 is carried around with the revolving shaft. When the armature- 

 lever g is pulled down as described, the lower end of I catches in 

 a second notch in its extremity and holds it down until the motor 

 is reset by hand. The two different positions of these levers can 

 be seen in the two views of Fig. 11. 



The most usual positions in which two portable series of 

 cameras were placed when used at the studio for making front 

 and rear views, or "foreshorteuings," in conjunction with the 

 first twelve of the permanent series for side views, or " laterals," 

 are shown in Fig. 1, at / and e, thirty-five feet, at E' and F', 

 forty feet, or at E'' and F", sixty feet from the centre of the track. 

 In the last two sets of positions the cameras pointed directly up or 

 down the track, at right angles to the laterals. Photographs made 

 in them were therefore termed "ninety-degree foreshortenings." 

 Other combinations of position were made by placing one of the 

 portable batteries of cameras at sixty degrees front or rear, with 

 the other at ninety degrees rear or front respectively. In the 

 ninety-degree foreshortenings, which were made especially to 

 analyze side oscillations of the body or limbs during locomotion, 

 it was desirable that the successive points of view in the series 

 should be all in the same vertical plane, and in order to have 

 them so the camera-box was stood on end, with the lenses directly 

 above one another. When the cameras were so placed the " hori- 

 zon line" was on a level with a point midway between lenses 6 

 and 7, and the first exposure of the series was made through the 

 lowest lens, except in a few special cases. 



The portable backgrounds used in all the photographing away 

 from the studio (except with a few of the wild animals at the 

 Zoological Gardens) were frames four metres long by three metres 



