THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
[March 22, 1873. 
about 1 fr. 20 c. a-day; most of them possess a little land. 
Two per cent, of their wages is kept back, and goes to the 
sick club fund, the capital of which at the beginning of 
>1871 exceeded 18,000 francs. The quantity of mercury 
produced at Vallalta during fifteen years—ranging from 
533 kilograms in 1856 to 34,776 kilograms in 1870— 
amounted to 324,856 kilograms, or nearly 320 tons. 
PROFESSOR TYNDALL ON LIGHT* 
(Continued from p. 730.) 
"The most familiar illustration of the interference of 
sound-waves is furnished by the beats in music, which 
■ are produced by two musical sounds slightly out of 
unison. When two tuning-forks which are in perfect 
■unison, are agitated at the same time, the two sounds pro¬ 
duced flow without roughness, as if. they were but one. 
But, by attaching to one of the forks a two-cent piece, 
we cause the loaded fork to vibrate a little more slowly 
than its neighbour. Suppose that one of them performs 
101 vibrations in the same time as the other performs 
100, and let us assume that at starting the condensa¬ 
tions and rarefactions of both forks coincide. At the 
101st vibration of the quickest fork they will again 
coincide, the quicker fork at this point having gained 
one vibration, or one whole wave upon the other. But 
a little reflection will make it clear that, at about the 
50th vibration, the two forks will be in opposition ; here 
the one tends to produce a condensation where the other 
tends to produce a rarefaction ; by the united action of 
'the two forks, therefore, the sound is quenched, and we 
have a pause of silence. This occurs where one fork has 
gained half a wave-length upon the other. At the 101st 
vibration we have again coincidence, and, therefore, 
augmented sound; at the 150th vibration, we have again 
a quenching of the sound. Here the one fork is three 
half-waves in advance of the other. With two forks so 
■circumstanced, we obtain those intermittent shocks of 
sounds separated by pauses of silence, to which we give 
the name of beats (such beats were rendered audible to 
all in the lecture). 
I now wish to show you what may be called the optical 
expression of those beats ; and here we have to fall back 
upon the fact that a luminous impression persists for a 
certain interval upon the retina. Attached to a large 
tuning-fork is a small mirror, which shares the vibra¬ 
tions of the fork, and on to the mirror is thrown a thin 
, beam of light, which shares the vibrations of the mirror. 
The fork is now still, and the beam reflected from it is 
received upon a piece of looking-glass, and thrown back 
- upon the screen, where it stamps itself as a small luminous 
disk. The agitation of the fork by a bow converts that disk 
into a band of light, and if you simply shake your heads 
to and fro you will be able to reduce that band to its 
■elements : you draw it, in fact, out to a sinuous line, thus 
•proving the periodic character of the motion which pro¬ 
duces it. By a sweep of the looking-glass we can also 
cover the screen from side to side by this luminous scroll, 
the depth of the sinuosities indicating the amplitude of the 
vibration. 
We now pass on to the optical illustration of these 
•beats. The large fork which we have just employed re¬ 
mains in its position ; but instead of receiving the beam 
reflected from it on a piece of looking-glass, it is received 
upon a second mirror attached to a second fork, and cast 
by it upon the screen. We now sound both forks, and 
both of them act in combination upon the beam. It is 
drawn out, as you see, as before, the band of light gra¬ 
dually shortening as the motion subsides. Finally, when 
the motion ceases we obtain the disk of light. Weio-htino- 
•* Abstract of a series of lectures delivered in the Cooper 
Institute, New York, and reported in the New York 
Tribune. 
one of the forks as we did before with a two-cent piece, 
we throw it out of unison with the other, and now' observe 
the screen. Sometimes the forks conspire, and then you 
have the band of light drawn out to its maximum length. 
Sometimes the forks oppose each other, and then you 
have the band of light diminished to a circle. Thus the 
beats which address the ear express themselves optically 
as the alternate elongation and shortening of the band of 
light. If I move the mirror of this second fork, you have 
a sinuous line, as before, dravm out upon the screen ; but 
the sinuosities are sometimes deep, and sometimes they 
almost disappear, thus expressing the alternate increase 
and diminution of the sound, the intensity of which is ex¬ 
pressed by the depth of the sinuosities. 
Every complete vibration of our tuning-fork produces a 
wave of sound, and, as all sounds travel with the same 
velocity through air, the more rapid the vibration the 
shorter are the sound-waves. The pitch of a sound is 
w'holly determined by the rapidity of the vibration, as 
the intensity is by the amplitude. The rise of pitch with 
the rapidity of the impulses may be illustrated by the 
syren, -which consists of a perforated disk rotating over a 
cylinder into which air is forced, and the end of which is 
also perforated. When the perforations of the disk coin¬ 
cide wdth those of the cylinder, a puff escapes ; and when 
the puffs succeed each other with sufficient rapidity, the 
impressions upon the auditory nerve link themselves to¬ 
gether to a continuous musical note. The more rapid the 
rotation of the disk the quicker is the succession of the 
impulses, and the higher the pitch of the note. Indeed, 
by means of the syren the number of vibrations due to 
any musical note, whether it be that of an instrument, of 
the human voice, or of a dying insect, may be accurately 
determined. 
In the undulatory theory, pitch is the analogue of 
colour. The waves of light have been measured, and it 
is found that the more refrangible the light the shorter 
are the waves v'hich produce it. The shortest U'aves of 
the visible spectrum are those of the extreme violet; the 
longest, those of the extreme red; while the other colours 
are of intermediate pitch or wave-length. The length of 
a w'ave of the extreme red is such that it would require 
36,918 of them placed end to end to cover one inch, while 
64,631 of the extreme violet waves would be required to 
span the same distance. 
Now the velocity of light, in round numbers, is 190,000 
miles per second. Reducing this to inches, and multiply¬ 
ing the number thus found by 36,918, we obtain the 
number of w'aves of the extreme red in 190,000 miles. 
All these waves enter the eye, and hit the retina at the 
back of the eye in one second. The number of shocks 
per second necessary to the production of the impression 
of red is, therefore, four hundred and fifty-one millions of 
millions. In a similar manner, it may be found that the 
number of shocks corresponding to the impression of 
violet is seven hundred and eighty-nine millions of mil¬ 
lions. All space is filled with matter oscillating at such 
rates. From every star waves of these dimensions move 
with the velocity of light like spherical shells outward. 
And in the ether, just as in the water—indeed, more 
truly than in the water—the motion of every particle is 
the algebraic sum of all the separate motions imparted to 
it. Still, one motion does not blot the other out; or, if 
extinction occur at one point, it is made good at soma 
other point. Every star declares by its light its own un¬ 
damaged individuality, as if it alone had sent its thrills 
through space. 
The principle of interference applies to the waves of 
light as it does to the waves of water and the waves of 
sound. And the conditions of interference are the same 
in all three. If two series of light-waves start at the 
same moment from a common origin, crest coincides with 
crest, sinus with sinus, and the two systems blend to¬ 
gether to a single system of double amplitude. If both 
series start at the same moment—one of them, however, 
being, at starting, a whole wave-length in advance of the 
