562 
Proceedings of the Royal Society 
ear itself, the oscillations of the plate I observed, and which were 
capable of causing a sensation of sound, would only amount, when 
they reached the hair cells, to about the y-giro o' °f an inch, and thus 
cause a movement of the auditory hair through about Jth of its 
possible amplitude of excursion. 
2 d Method — The movements may also be observed by throwing 
obliquely a beam of light on a reflecting surface on the disk, and 
catching the reflected ray on a screen. I did not find this method 
so satisfactory as the one just described. 
3 d Method . — I cemented the disk of the distal telephone to the 
thin membrane of one of Koenig’s manometric capsules, then there 
was no difficulty in observing the oscillations of the flame in a 
rapidly revolving mirror, when a strongly vibrating tuning-fork was 
placed opposite the proximal telephone. I failed, however, in 
seeing any movements produced by speech. The method, how- 
ever, is capable of more refined application than the means at 
present at my disposal will allow. 
4 th Method . — I have attempted to record the movements of the 
disk graphically by attaching to it a lever bent at right angles at 
one end, bringing the point on a rapidly moving surface. Ko 
oscillations could be detected. Such oscillations were, however, 
recorded when, instead of a disk, I used forks at the proximal and 
distal ends, as already described. The movements of the distal 
fork were then recorded, but it was observed that the sound of the 
fork was audible long after all oscillations could be recorded, show- 
ing that the movements of the fork were too delicate to be recorded 
by this method. 
5. A Mode of Intensifying the Sound of the Telephone . — In study- 
ing the transmission of sounds of various kinds, I had occasion to 
use a rapidly revolving wheel opposite the proximal telephone. 
The wheel in my possession, which moves with greatest velocity is one 
about 4 inches in diameter, having placed transversely on its circum- 
ference twenty-four rectangular bars of iron. The wheel is driven by 
two electro-magnets, and the current employed was obtained from 
twenty of Sir William Thomson’s tray cells. With this power it 
performs about eighty revolutions per second. When the proximal 
telephone was brought near the circumference of the wheel, the 
distal telephone sounded so loudly that a rattling sound like that 
