152 Dr. Barton and Mr. Garrett on Vibration-Curves 



by indiarubber cords, the place of exposure being that of 

 practically zero acceleration. 



Consider now the second portion of light which passes 

 through the pin-hole. It next falls upon a lens, then upon 

 the concave mirror of the optical lever. This sends it to the 

 photographic plate just below the image of slit and string. 

 The adjustment of the single lens is in this case sufficient for 

 focussing. 



Hence, if all were still, we should have upon the upper 

 half of the plate a bright image of the vertical slit crossed by 

 the string's shadow, while below would be the bright spot 

 from the optical lever. When the string is excited, its shadow 

 moves up and down the slit, and the belly moving in response 

 the bright spot also oscillates vertically. When, however, 

 the plate is moving horizontally, we have both oscillations 

 drawn out into displacement-time curves. The upper one, 

 for the string, is, in the positive print, a black line on a white 

 ground ; the lower one, for the belly, being a white one on a 

 dark ground. Lumiere X-ray plates were found very suit- 

 able and were used almost throughout. The plates travelled 

 in the rails at about eleven feet per second, the length (four 



inches) passing in about ^ of a second. Consequently the 



exposure of the plate to the spot (if at rest) bore the same 



ratio to oo of a secund that the spot's diameter bore to the 



plate's length. For a rather large spot, of 1 mm. diameter, 



the exposure was therefore of the order „-hqq of a second. 



When, however, the spot was tracing an oblique line at an 

 angle (j> with the horizontal, the exposure was only (cos (f))~ 

 times the above. In some cases where the spot was rather 

 small, Q'8 mm. diameter, and the obliquity considerable, the 



exposure was only . . of a second. 



The magnifications of the motions upon the actual nega- 

 tives (4 inches x 3) are for the string 2' 9 and for the belly 

 760. Thus in the prints, to any scale, the motion of the belly 

 is magnified about 260 times in comparison with that of the 

 string. In some of the negatives, motions of the belly of 



the order - Q00 mm. and accomplished in the .-^^ second, 



are clearlv traceable and occur regularly in each wave (see 

 figure 8, PL L). 



As to sense and direction of the motion, on the prints the 

 beginning of the time is at the left side and the ordinates are 

 all inverted, i. e., an upward motion of either shadow or spot 

 means a downward motion of string or belly. 



