﻿Vibration- Curves of String and Air. 577 



recorded by placing a thin animal membrane over one of the 

 centre holes of the instrument, which had three on each side. 

 In the centre of this membrane was attached by shellac and 

 a strip of membrane one end of an aluminium stalk, whose 

 other end was in like manner attached to a small plane 

 mirror by Hilger. This mirror rocked upon a horizontal 

 axis, being mounted on a sewing-needle clipped to small brass 

 brackets fastened on the vertical face of the sound-box. 

 Now the mirror received, through a pin-hole in a sheet of 

 tinfoil, the light from an electric arc-lamp, which, after 

 reflexion and passing through a lens, was focussed upon the 

 upper part of the sensitive plate about o* feet distant. 



Thus as the membrane moved, owing to the aerial motions 

 responsive to the string's vibrations, the mirror rocked on 

 its axis and so caused the spot of light to rise and fall on the 

 photographic plate. The composition of this vertical periodic 

 motion of the spot with the uniform horizontal motion of the 

 plate gave automatically the displacement and time-curve 

 sought, which in the positive prints and reproductions appears 

 as a white line on a black ground. 



The attachment of the aluminium stalk to the minor was 

 at less than 1 mm. from its axis, the absolute magnification 

 of the diaphragm's motion working out to about 4200 for 

 the actual negatives (4|- in. x 3j in.). The magnification of the 

 string's motion is about 3 times only. Hence on any repro- 

 duction the relative magnification of diaphragm's and string's 

 motions remains at the same constant value of 1400 times 

 nearly. 



The " string " used is of steel 0'92 mm. diameter, and the 

 distance between the bridges is 100 cm. 



On the positive prints the beginning of the time is at the 

 left, the string's motion is represented inverted by its shadow 

 owing to the presence of the lens which focussed the shadow 

 on the plate. In the white wavy line which records the 

 motion of the diaphragm a crest indicates a motion of the air 

 outwards from the interior of the sound-box and vice versa. 



Results. 



The results obtained are illustrated by the 39 figures on 

 Plate XIX., each figure containing two curves showing the 

 simultaneous motion of string and air. 



All through, the string was illuminated at points between 

 47 and 50 cm. from one end, usually at or near 49 cm. The 

 frequency of the string for the figures 1-10 was 128 per sec. 



Figure 1 is from the first exposure made under the present 

 conditions, the string was bowed at one-tenth its length. 



