254 



Professor Edwin H. Barton 



[March 8, 



its centre. The real image of this sht, crossed by the shadow of the 

 string, was then focused by a second lens on to a photographic plate 

 in a dark room. This plate was shot along horizontal rails by elastic 

 cords which were just slack when the plate received the light. Thus 

 the plate moved uniformly and horizontally, while the shadow of the 

 vibrating string showed its special motion vertically. The corre- 

 sponding motions of bridge, belly, or air were obtained on the same 

 photographic plate by the light reflected from a tiny rocking mirror 



Fig. 3. — Monochokd Apparatus. 



whose slight tilt was produced by the motion of the part under test. 

 (The principle of this experimental method was then demonstrated, 

 the humped form of the curve due to phicking the string and the 

 two-step zig-zag produced by careful bowing being shown.) Fig. 8 

 g'ves a diagram of the method for the monochord, also a detail of 

 the rocking mirror for the bridge's motion. Fig. 4 shows photo- 

 graphic traces for the monochord string and belly. The two curves 



