388 A. M. Mayer—Method of Detecting the Phases of 
describe only two; the first, though impracticable, I speak of 
to render clear the general method of all; the second I give on 
account of its simplicity, ease of execution, and the superior 
accuracy of its numerical results. : 
Take two tuning forks giving the same note and having 
mirrors attached to their similar prongs; place one at A, the 
other anywhere on the line OX. Reflect a pencil of light from 
each mirror of the forks to a revolving mirror, whose axis of 
rotation is in a plane parallel to the planes of vibration of the 
forks. If the fork B, which vibrates sympathetically, be 
placed at B*, B*, BY, ete, then the two pencils reflected from 
the forks will, on striking the revolving mirror, be drawn into 
two sinuous curves, and the flexures of the two curves will 
run parallel to each other, that is, the curves will appear as 
the two rails of a sinuous railway; but, if the fork B be placed 
at B', B*, or B®, ete, then the sinuosities of the two curves 
will no longer be parallel but will be opposed; that is, where 
a flexure of one of the curves is concave on the left, the 
opposite flexure of the other curve will have its concavity 
on the right. If the fork B be placed at intermediate positions, 
in reference to those above stated, we will have neither concore 
ance nor opposition of the flexures, but intermediate eth 
depending on the fraction of half wave-lengths at which the 
sympathetic fork is placed on the line OX. F 
t is now readily seen that if we should place the fork B . 
two successive points, as B? and B‘, on the line OX, so tha 
exact concordance of flexures of the curves should be seen . 
each of these points, then evidently we have placed the fork . 
two positions removed from each other by a wave-length, for at 
these points the air had at the same instant the same phase © 
vibration. Thus we have measured a wave-length. pier 
more, if by any means we could move the fork B around 
so that during this motion it always preserved, in reference 10 
A, the same relation of vibratory phase, we would have deter- 
mined the form of the wave-surface produced by the propas® 
tion of A’s vibrations. 
The above is an exposition of the thoughts that have ste 
ied my mind for several months, and they ultimately led to t : 
ollowing method, by which a!l I have narrated can be accom 
plished without any difficulty ; thanks to the inventive pouty 
of Mr. Konig, to whose skillful aid so many physicists are CO? 
tinually indebted. : ‘ch 
The membranes of Mr. Kénig’s manometric capsules ede 
us with surfaces which vibrate in perfect accordance with the 
air which touches them, and we can lead the impulses of ae 
membranes through gum tubes to gas jets waded | at any desir 
oint, where the vibrations of their ees 
, ean be com 
Thus they are far superior to the tuning forks, which 
require ee 
