May 10, 1873.] 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
88 T 
groups of rays; according to Kirchhoff’s principle those 
vapours if crossed by solar light ought to absorb rays of 
the same refrangibility as those which they emit. He 
proved this to be the case ; he was able by the interposi¬ 
tion of a vapour to cut out of the solar spectrum the 
band corresponding in colour to that vapour. Now, the 
sun possesses a photosphere, or vaporous envelope; doubt¬ 
less mixed with violently agitated clouds; and Kirchhoff 
saw that the powerful rays coming from the solid or the 
molten nucleus of the sun must be intercepted by this 
vapour. One dark band of Fraunhofer, for example, 
occurs in the yellow of the spectrum. Sodium vapour is 
demonstrably competent to produce that dark band; 
hence Kirchhoff inferred the existence of sodium vapour 
in the atmosphere of the sun. In the case of metals 
which emit a large number of bands, the absolute coinci¬ 
dence of every bright band of the metal with a dark 
Fraunhofer line raises to the highest degree of certainty 
the inference that the metal is present in the atmosphere 
of the sun. In this way solar chemistry was founded on 
spectrum analysis. 
I have now to make plain to you if I can, through the 
analogy of sound, the physical meaning of emission and 
absorption. I draw a fiddle-bow across this tuning-fork 
and it immediately fills the room with a musical sound; 
this is the radiation or emission of sound from the fork. 
A few days ago, on sounding this fork, I noticed that 
when its vibrations were quenched, the sound seemed to 
be continued, though more feebly; the sound appeared 
to come from under the table, where stood a number of 
tuning-forks of different sizes and rates of vibration. 
One of these, and one only, had been started by the fork, 
and it was one whose rate of vibration was the same as 
that of the fork which started it. This is an instance of 
the absorption of the sound of one fork by the other. 
Placing two forks thus near each other, sweeping the bow 
over one of them and then quenching the agitated fork, 
the other continues to sound. Placing a cent piece on 
each prong of one of the forks we destroy its perfect 
synchronism with the other, and then no communication 
of sound from the one to the other is possible. I will now 
do with light what has been here done with sound. 
Placing a tin spoon containing sodium in a Bunsen’s flame, 
we obtain this intensely yellow light, which corresponds 
in refrangibility with the yellow band of the spectrum. I 
will send the white light from our lamp through that flame, 
and prove before you that the yellow flame intercepts the 
yellow of the spectrum, producing to all intents and 
purposes a dark Fraunhofer’s band in the place of the 
yellow. 
Mentally as well as physically every age of the world 
is the outgrowth and offspring of all preceding ages. 
Science proves itself to be a genuine product of nature 
by growing according to this law. We have no solution 
of continuity here. Every great discovery has been duly 
prepared for in two ways; first, by other discoveries 
which form its prelude, and secondly, through the sharp¬ 
ening by exercise of the intellectual instrument itself. 
Thus Ptolemy grew out of Hipparchus, Copernicus out 
of both, Kepler out of all three, and Newton out of all 
the four. Newton did not rise suddenly from the sea 
level of the intellect to his amazing elevation. At the 
time that he appeared the table land of knowledge was 
already high. He juts, it is true, above the table land 
as a massive peak; still he is supported by it, and a 
great part of his absolute height was the height of 
humanity in his time. It is thus with the discovery of 
Kirchhoff. Much had been previously accomplished ; 
this he mastered, and then by the force of individual 
genius went beyond it. He replaced uncertainty by 
certainty, vagueness by definiteness, confusion by order ; 
and I do not think that Newton has a surer claim to the 
discoveries that have made his name immortal than 
Kirchhoff has to the credit of gathering up the frag¬ 
mentary knowledge of his time, of vastly extending it, and 
of infusing into it the life of great principles. Splendid 
results have since been obtained, in relation to which both 
England and America have played an honourable part 
but, splendid as they are, they are but the sequel and 
application of the principles established in his Heidelberg 
laboratory by the German investigator. 
I have now come almost to the end of my task in this 
city, and, indeed, in America. My desire has been to 
show you, with as little break of continuity as possible,, 
the past growth and present aspect of a department of 
science, in which have laboured some of the greatest- 
intellects the world has ever seen. My friend, Professor 
Henry, in introducing me at Washington, spoke of me as 
an apostle ; but the only apostolate that I intended to 
fulfil was to place, in plain words, my subject before you, 
and to permit its own intrinsic attractions to act upon 
your minds. In the way of experiment, I have tried to 
place before you the best which, under the circumstances,., 
could be provided ; but I trust that each experiment has 
had a distinct intellectual value, for experiments ought 
to be the representatives and expositors of thought—a 
language addressed to the eye as spoken words are to 
the ear. In association with its context, nothing is more 
impressive or instructive than a fit experiment ; but,, 
apai't from its context, it rather suits the conjurer’s pur¬ 
pose of surprise than that purpose of education which 
ought to be the ruling motive of the scientific mind. 
And now a brief summary of our work will not be out 
of place. Our present mastery over the laws and phe¬ 
nomena of light has its origin in the desire of man to 
know. We have seen the ancients busy with this 
problem, but, like a child who uses his arms aimlessly for 
want of the necessary muscular exercise, so these early 
men speculated vaguely and confusedly regarding light, 
not having as yet the discipline needed to give clearness 
to their insight, and firmness to their group of principles. 
They assured themselves of the rectilineal propagation of 
light, and that the angle of incidence was equal to the 
angle of reflection. For more than a thousand years—I 
might say, indeed, for more than fifteen hundred years 
subsequently—the scientific intellect appears as if smitten 
with paralysis, the fact being that, during this time, the 
mental force, which might have run in the direction of 
science, was diverted into other directions. 
The course of investigation as regards light was resumed 
in 1100 by an Arabian philosopher named Alhazan. 
Then it was taken up in succession by Roger Bacon, 
Vitellio, and Kepler. These men, though failing to detect 
the principle which ruled the facts, kept the fire of in¬ 
vestigation constantly burning. Then came the funda¬ 
mental discovery of Snell, that corner-stone of optics, as I 
have already called it, and immediately afterward we have 
the application by Descartes of Snell’s discovery to the 
explanation of the rainbow. Then came Newton’s crown¬ 
ing experiments on the analysis and synthesis of white 
light by which it was proved to be compounded of various 
kinds of light of different degrees of refrangibility. 
In 1676 an impulse was given to optics by astronomy. 
In that year Olaf Roemer, a learned Dane, was engaged 
at the Observatory of Paris in observing the eclipses of 
Jupiter’s moons. He converted them into so many 
signal-lamps, quenched when they plunged into the 
shadow of the planet, and relighted when they emerged 
from the shadow. They enabled him to prove that light 
requires time to pass through space, and to assign to it 
the astounding velocity of 190,000 miles a second. Then, 
came the English astronomer, Bradley, who noticed that 
the fixed stars did not really appear to be fixed, but 
described in the heavens every year a little orbit resem¬ 
bling the earth’s orbit. The result perplexed him, but- 
Bradley had a mind open to suggestion, and capable of 
seeing, in the smallest fact, a picture of the largest. He 
was one day upon the Thames in a boat, and noticed that 
as long as his course remained unchanged the vane upon 
his masthead showed the wind to be blowing constantly 
in the same direction, but that the wind appeared to vary 
•with every change in the direction of his boat. “ Here,’ 
