CAPILLARY RIPPLES. 53 
lightest and softest tap ripples are produced, but you cannot see them, 
the dust on the surface appears stationary. Now I will ask you to 
blink your eyes and get, if possible, an instantaneous view of the screen, 
you will then see that there are circular ripples round that point; but 
to make them easily observed I will put this motor in the beam of 
light. The motor is now going, and as soon as the speed is right the 
ripples will appear perfectly stationary, and so conspicuous that it is 
difficult to imagine that under ordinary circumstances they are quite 
invisible (applause). 
While the fork is in position I will illustrate the fact that vibrations 
which are slower make ripples which are larger and which travel more 
slowly. I am now placing in position another fork which gives a lower 
note, and you will see that the smaller number of vibrations produces 
evidently the larger waves. 
The next thing I will show is that the two forks are correctly 
tuned to an octave. As the motor changes its speed the ripples 
produced by each creep in and creep out together, or are both 
at rest. Next when they are creeping in or out it must be evident 
to every one that the little ripples are travelling more quickly 
than the bigger ones (applause). I shall now take two forks which 
give exactly the same note. You see the ripples due first to one and 
then to the other separately ; they are the same size and travel at the 
same speed. But on sounding the two together there is a sort of 
pattern rather like the back of a watch due to the combined ripples, 
and the lines of intersection are stationary even though the ripples are 
moving. Ifthe intersection lines are always in the same place they 
should be visible by continuous light. I take away the motor and you 
see stationary ripples of half the wave length due to the interference of 
the two sets travelling in opposite directions. Those are not the true 
ripples; they are an interference phenomenon due to the two sets 
independently travelling (applause). I will next show you a curious 
thing. If one fork is put out of tune by the addition of a small 
piece of wax the two sets of ripples still interfere with one another, but 
the waves produced by the slightly lower fork (that with the added 
wax) do not travel quite so fast, and so the lines of interference slowly 
change. The result is that when the forks both sound the interference 
pattern travels slowly from the sharper to the flatter fork, these inter- 
ference ripples are visible by continuous light. Similarly with two 
forks, one giving the octave of the other stationary interference, ripples 
can be produced as is the case now, or if the interval is imperfect they 
slowly creep towards the fork that is too low. 
I wish next to show by means of a little piece of microscope cover glass 
cut straight on one side and with a concave curve on the other, thatripples 
may be used to illustrate the phenomena of optics and acoustics, reflec- 
tion, diffraction and the “Doppler principle.” The glass is floating on the 
mercury and as soon as it is quiet I hope to be able to show the true re- 
flection from the concave surface and the formation of an image at the 
focus. I will next use the straight edge. With this the reflection is 
exactly that which would be produced if there were another identical 
tuning fork on the other side of the glass. The result is the same inter- 
