730 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
[March 15, 1873. 
clearly seize the image of the moving wave you 'will see 
that every particle of water along the front of the wave 
is in the act of rising, while every particle along its back 
is in the act of sinking. The particles in front reach in 
succession the crest of the wave, and as soon as the crest 
is passed they begin to fall. They then reach the furrow 
or sinus of the wave and can sink no farther. Immedi¬ 
ately afterwards they become the front of the succeed¬ 
ing wave, rise again until they reach the crest, and 
then sink as before. Thus while the waves pass onward 
horizontally, the individual particles are simply lifted up 
and down vertically. Observe a sea-fowl, or, if you are 
a swimmer, abandon yourself to the action of the waves ; 
you are not carried forward, but simply rocked up and 
down. The propagation of a wave is the propagation of 
a form, and not the transference of the substance which 
constitutes the wave. 
The length of the wave is the distance from crest to 
crest, while the distance through which the individual 
particles oscillate is called the amplitude of the oscilla¬ 
tion. You will notice that in this description the parti¬ 
cles of water are made to vibrate across the line of propa¬ 
gation. 
And now we have to take a step forward, and it is the 
most important step of all. You can picture two series 
of waves proceeding from different origins through the 
same water. AYhen, for example, you throw two stones 
into still water, the ring-waves proceeding from the two 
centres of disturbance intersect each other. Now, no 
matter how numerous these waves may be, the law holds 
good that the motion of every particle of the water is the 
algebraic sum of all the motions imparted to it. If crest 
coincide with crest, the wave is lifted to a double height; 
if furrow coincide with crest, the motions are in opposi¬ 
tion, and their sum is zero. We have then still water, 
which we shall learn presently corresponds to what we 
call darkness in reference to our present subject. This 
action of wave upon wave is technically called inter¬ 
ference, a term to be remembered. 
Thomas Young’s fundamental discovery in optics was 
that the principle of Interference applied to light. Long 
prior to his time an Italian philosopher, Grimaldi, had 
stated that, under certain circumstances, two thin beams 
of light, each of which, acting singly, produced a lumi¬ 
nous spot upon a white wall, when caused to act 
together partially quenched each other and darkened 
the spot. This was a statement of fundamental signifi¬ 
cance, but it required the discoveries and the genius of 
Young to give it meaning. How he did so I will now try 
to make clear to you. You know that air is compres¬ 
sible ; that by pressure it can be rendered more dense, 
and that by dilatation it can be rendered more rare. 
Properly agitated a tuning-fork now sounds in a manner 
audible to you all, and most of you know that the air 
through which the sound is passing is parcelled out into 
spaces in which the air is condensed, followed by 
other spaces in which the air is rarefied. These con¬ 
densations and rarefactions constitute what we call 
waves of sound. You can imagine the air of a room 
traversed by a series of such waves, and you can imagine 
a second series sent through the same air, and so related 
to the first that condensation coincides with condensation 
and rarefaction with rarefaction. The consequence of 
this coincidence would be a louder sound than that pro¬ 
duced by either system of waves taken singly. But you 
can also imagine a state of things where the condensa¬ 
tions of the one system fall upon the rarefactions of the 
other. In this case the two systems would completely 
neutxalize each other. Each of them taken singly pro¬ 
duces sound j both of them taken together produce no 
sound. Thus, by adding sound to sound we produce 
silence, as Grimaldi in his experiment produced dark¬ 
ness by adding light to light. 
The possible analogy between sound and light here at 
once flashes upon the mind. Yoxmg generalized this ob¬ 
servation. He discovered a multitude of similar cases, 
and determined their precise conditions. On the as¬ 
sumption that light was wave-motion, all his experiments, 
on interference were explained; on the assumption that 
light was flying particles, nothing was explained. In. 
the time of Huyghens and Euler a medium had been as¬ 
sumed for the transmission of the waves of light; but 
Newton raised the objection that, if light consisted of the 
waves of such a medium, shadows could not exist. The 
waves, he contended, would bend round opaque bodies 
and produce the motion of light behind them, as sound- 
turns a corner, or as waves of water wash round a rock.. 
It was proved that the bending round referred to by 
Newton actually occurs, but that the inflected waves 
abolish each other by their mutual interference. Young* 
also established a fundamental difference between the 
waves of light and those of sound. Could you see the 
air through which sound-waves are passing, you would 
observe every individual particle of air oscillating to and 
fro in the direction of propagation. Could you see the- 
ether, you would also find every individual particle 
making a small excursion to and fro ; but here the mo¬ 
tion, like that of the water-particles above referred to, 
would be across the line of propagation. The vibra¬ 
tions of the air are longitudinal, the vibrations of the 
ether are transversal. 
It is my desire that you should realize with the utmost 
possible clearness the propagation of waves, both in 
ether and in air. And with this view, I bring before 
you an experiment wherein the air particles are repre¬ 
sented by small spots of light. These spots are parts of 
spirals (drawn upon a circle of blackened glass, and when 
the circle is caused to rotate, the spots move in successive 
pulses over the screen. You have here clearly set be¬ 
fore you how the pulses travel incessantly forward^ 
while the particles that compose them perform any oscil¬ 
lation to and fro. We have in this case the picture of a. 
sound wave, in which the vibrations are longitudinal. 
By another arrangement of our glass wheel, we produce- 
an image of a transverse wave, and here we observe the. 
waves travelling in succession over the screen, while 
each individual spot of light performs an excursion to- 
and fro across the line of propagation. 
{To he continued.') 
THE BROUGH FUND. 
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Subscribers will be informed in the columns of this. 
Journal and the Chemist and Druggist of the arrange¬ 
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