458 



Prof. Barton and Mr. Ebblewhite on 



The respective magnifications in the two cases were as 

 follows : — 



Part of Bridge 



under 

 Examination. 



Actual Magnifications 

 on quarter-plate negative. 



Ratio of 

 Magnifications 

 on any print. 



String's 

 Motion. 



Bridge's 



Motion. 



E-string Corner 



J 2-2 



J 2-1 



i 



598-3 

 603 



271 



287 



Gr-string Corner 





Longitudinal Motion of Bridge. — For the motions of the 

 E-string corner of the bridge lengthwise of the strings, 

 the arrangements adopted are shown diagrammatically in 

 fig. 1 (PL VII.) and in perspective from a photograph in 

 fig. 2 (PL VII.). 



Referring to fig. 1, the arc lamp and condenser are indi- 

 cated by A L, and C, from which part of the beam passes 

 direct to H, a screen pierced by a small hole. The light 

 from H reaches a concave mirror m on the optical lever, and 

 is thence converged to the point R on the sensitive plate PP, 

 which is shot along the rails INA in the dark room. This 

 gives on the lower part of the plate a record of the bridge's 

 motion, which in the positive prints appears as a white line 

 on a dark ground. 



Let us now follow the other part of the beam from the 

 arc lamp. This is reflected by the pair of plane mirrors Mx M 2 

 to the vertical slit S, from which the light falls upon the 

 lens L 1? which focusses a real image of the slit on the string- 

 in use at 12 or 13 cm. from the nut, 32 cm. being the 

 length of the vibrating portion of the strings from nut 

 to bridge. The light is then reflected by the plane mirror M 

 to the lens L 2 , which gives on the plate at R a real image of 

 the slit crossed by the shadow of the string ; it accordingly 

 leaves a record of the string's motion, which in the positive 

 prints is a dark line on a light ground. This image is 

 on the upper part of the plate, precisely over that from the 

 optical lever m. Thus a comparison of the two records 

 gives information as to the relative phases of the vibrations 

 of string and bridge. 



Let us now examine more closely how the bridge's motion is 

 recorded. Fig. 3 (PL V J I.) shows on a larger scale the bridge 

 of the violin with the optical lever somewhat out of place, 



