obtained from a Monochord Sound-box and String. 151 



with its legs over the table legs. The optical arrangements 

 were placed on a second table at right angles to the first. 

 The space between the bridges is 100 cm. and the "string" 

 used throughout was a steel wire 0*92 mm. in diameter. 



Helmholtz analysed the motion of a bowed string by means 

 of the vibration microscope, thence deriving the displacement- 

 time curve. Krigar-Menzel and Raps focussed upon the 

 string the real image of an illuminated slit. This image was 

 at right angles to the string, and was used to form a second 

 real image upon the moving photographic film carried by a 

 rotating drum. This method gave a direct photograph of the 

 displacement-time curve, for the shadow of the string moved 

 up and down along the bright image of the slit as the drum 

 carried the film along horizontally. 



For the string's motion we adopted a modification of the 

 method of Krigar-Menzel and Raps ; their film on a drum 

 being replaced in our experiments by an ordinary glass nega- 

 tive. This was shot horizontally along rails like the smoked 

 plate in the Electric Chronograph due to the Rev. F. J. Smith 

 of Oxford. 



But the motion of the belly needed providing for also. 

 This was detected by a small three-legged optical lever made 

 of aluminium. Two of its feet formed its axis of rotation 

 and were carried by a hole and slot in a plate mounted on a 

 massive stand. The third leg, to detect the motion of the 

 belly, rested upon a plane, thus completing the geometrical 

 arrangement of "hole, slot, and plane." The lever was held 

 down by indiarubber bands. The " plane " consisted of the 

 head of a drawing-pin, bare or covered with glass, or a glass 

 plate cemented on to the belly direct. These separate cases 

 will, be dealt with in connexion with the experiments 

 themselves. 



The source of light was an arc lantern with a condenser 

 five inches in diameter. This converged the beam upon a 

 vertical slit and pin-hole in a sheet of tinfoil mounted on a 

 glass plate. Let us first trace the course of the light passing 

 through the slit. It is focussed by a lens upon the string 

 near, but not quite at, its middle. It next passes on to a 

 plane mirror behind the monochord. The beam then returns 

 close by the side of the lantern, through a hole in the wall 

 into another room where it reaches the photographic plate. In 

 this part of its path a second lens serves to focus upon the plate 

 the bright image of the slit crossed at its middle by the black 

 shadow of the string. The second room containing the rails 

 and photographic plate is, of course, kept pitch dark during 

 an exposure. The carriage containing the plate is shot 



