54, CAPILLARY RIPPLES. 
ference that we obtained with two forks is next produced with one. Next 
if the tray containing the liquid is made to slowly swing like a pendu- 
lum, as it now is, then the image of the point which is behind that reflector 
is moving backwards and forwards also. Now here one set of waves 
is moving in the same direction as the mercury and the other in the 
opposite direction, so we have an illustration of Déppler’s principle. 
Ifa railway engine is approaching the whistle sounds sharp, if it is 
receding it sounds flat. Now returning to our experiment, when the 
reflector is approaching the style the image of it, the imaginary style, 
is approaching also and is, therefore, sharp, and the interference 
pattern between the two, since one note is sharpened and the other 
flattened, moves from the sharp to the flat note, and as the mercury 
moves alternately each way the interference pattern swings backwards 
and forwards also. I next wish to show that waves go round a corner. 
Light ordinarily to all intents and purposes casts a perfectly sharp 
shadow and does not go round a corner. It is not true in reality ; 
light does go round a corner, but this can only be observed under 
special conditions. All wave action goes round a corner and true 
sharp shadows are impossible. On placing the piece of glass in the 
way of the ripples it is, I think, manifest that it does not cast a sharp 
shadow. 
Then it is possible to make plane waves by the simple device of re- 
placing the glass point by means of a glass straight-edge. This is now 
done, and I shall be able with them to show the formation of a focus 
far more conspicuously than. before. 
I have endeavoured this afternoon, but I am afraid with imperfect 
success, as the result of the difficulties to which I have been subjected, to 
illustrate experimentally the laws which are due to the formation of 
ripples by tuning-forks. But there is yet another way of producing 
these ripples, which is simply to take a tube, to attach a glass style to a 
piece of skin tied over the end of the tube, and then to set that tube so 
that the point just touches the surface of the liquid. What Iam going to 
try, and may succeed in showing, is that the ordinary tone of the voice 
is sufficient to set the air vibrating and with it pieces of paper or 
skin or anything else; and a fine glass style attached to the skin 
stretched across the end of the tube will vibrate according to the 
tone of the voice. If, therefore, it is possible to speak in a tone of 
voice that corresponds with the note that the motor is giving, then at 
the time of speaking the vibrations of the voice become perfectly 
evident; if I speak in a tone exactly corresponding to the motor the 
rings do not move at all. But if I raise my tone in the slightest 
degree (speaking in a slightly higher tone)the vibrations are faster, and 
under those circumstances the rings begin rushing out (applause) ; and, 
on the other hand, if I lower my voice ever so little (very slightly 
dropping his voice) it is quite enough and they begin rushing in. In 
fact I cannot help thinking that if this arrangement were neatly and 
conveniently put up it would form an excellent and clear method of 
demonstrating whether a person could sing in tune or not; he would 
simply be told to sing the particular note put before him, and if he 
eould sing it correctly these rings would not move at all; but a minute 
