416 PROF. J. C. BOSE ON THE 
is increased by increasing the distance of the point of attachment 
of the thread on the long arm, away from the fulcrum; it is 
also increased by increasing the distance of the recording drum 
from the lever. The vertical movement of the spot of light 
is converted into a horizontal movement through a second 
reflection in a suitably inclined mirror, M. 
These curves enable us to obtain the absolute value of the 
movement executed by the tip of the leaflet. The abscissa of 
the curve represents the time, which is known from the speed 
Fig. 8. 
Optical lever for mechanical! record. 
of rotation of the drum. The absolute value of the ordinate, 
representing the distance travelled by the leaf, is found by 
moving the long arm of the lever through a given vertical 
distance, say one centimetre, and observing the displacement of 
the reflected spot of light on the recording drum. 
Simultaneous Mechanical and Electrical Record. 
In order to demonstrate the mutual relation of the two 
responses, it was necessary to take two records simultaneously 
on the same recording drum (fig. 9). 
