﻿Vibration Curves for Strings and Air of Violin. 887 



arranged to have no tension (and so produced no acceleration) 

 when the plate passed the opening and secured the exposure. 



This method left little or nothing to be desired when the 

 strings were bowed, as that excitation sustained the motions 

 till the photograph was taken. Moreover, with a bowed 

 note the sustained motion is of chief interest. 



But this method did not succeed so well for transitory 

 effects. Thus, in attempting to photograph the vibrations 

 excited by plucking or striking the records obtained were 

 repeatedly disappointing. The string's motion was often of 

 considerable amplitude, while the spot of light from the 

 mirror connected to the membrane scarcely stirred. This 

 was the more surprising since on observing the spot on a 

 screen in place of the plate, it appeared to have a transitory 

 motion of distinctly appreciable amplitude. It therefore 

 seemed as though after plucking the string the vibrations 

 of the air were at first large, but died away before the plate 

 could catch them by the ordinary method of shooting. 



After trying several ways designed to record this large 

 initial vibration (or displacement), the following method 

 seemed best and was adopted. The plate was placed in the 

 rails but kept in the dark till about a second before the 

 string was plucked or otherwise excited. When this 

 preliminary signal was given the plate was slid into the 

 exposure position by hand and there held against the tensions 

 of the elastic cords. Immediately the string was excited the 

 plate was released. The very tight elastic cords attached to 

 it (for this method of shooting) then gave it the besired 

 acceleration and revealed the special initial features of the 

 motions. 



But since on this plan a pause occurred with the plate in 

 position and exposed to stray light, it was necessary to use 

 the pierced black diaphragms D„ D 2 , D a , fig. 1, PI. XIII., and 

 black velvet screens V, V, V, V, V, V, to prevent the fogging 

 of the plate. 



Magnifications. — The string was usually illuminated by 

 the real image of the slit at 5 cm. to 12 cm. from the nut 

 (L e., end of the string remote from the bridge). On the 

 original negatives and prints the magnification of the motion 



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of this part of the string is obviously L 2 P-r-NL 2 . These 

 values were 139 and 36 cm. respectively and give a quotient 

 of 3*86 nearly. 



The magnification of the membrane's motion is double the 

 quotient : mP divided by the arm of the attachment of 

 stalk to mirror. Now mP, the distance from mirror to plate, 

 was 160 cm., and the small arm on which the stalk acted 



