4 
- 
2 
| 
with radiation. 
_ while I attempt to make clear to you all the strange facts 
ficial coloration of wine. Mr. Sorby,on the other hand, 
has given his attention to beer; so that you see, if I have 
been taking you occasionally to the stars, I sometimes 
have the opportunity of travelling a great deal nearer | 
home. 
Mr. Sorby has also made some extremely delicate and 
interesting researches on the colouring matters existing in 
leaves. He has been able to identify numerous colouring 
principles, which he has arranged in five distinct groups : 
these groups rejoice in the names of chlorophyll, xanto- 
phyll, erythrophyll, chrysophyll, and phaiophyll, the ab- 
sorption spectra of which are perfectly distinct and well 
marked. It is found generally that leaves contain colours 
belonging to several groups, and frequently more than one 
of the same group. Mr. Sorby also finds that the change 
of colour which takes place in autumn consists chiefly in 
the disappearance of the chlorophyll, which renders the 
remaining colours visible, and these most frequently are 
of a yellowish tint. Some leaves, however, turn red in 
the autumn: this appears to be due to a falling off of the 
vital power of the plant, for by artificially diminishing the 
vital power, the intensity of this red colour is increased. 
One great value of this method of research is that it 
enables us to recognise special colouring-matters, even 
when mixed with several others, and to determine the 
particular conditions in which they occur in plants or 
NATURE 
II 
animals—whether in a solid state or in solution —and 
| whether those dissolved out by reagents exist as such in 
| the living organisms, or are the products of decom- 
positions. 
So that you see, on the whole, at the present moment, 
I think we may be full of hope that the new process may 
gradually lead to many more practical applications ; but 
really we cannot say much about them at present, because 
.the introduction of spectrum analysis is so recent. We 
are, however, already furnished with another instance of 
the close connection there always must be between any 
great advance in physical inquiry and the application of 
the skill of our opticians to aid usin the inquiry. We 
have the Sorby-Browning spectrum microscope, and then 
a large number of people can study the beautiful pheno- 
mena which this new method of research has opened up 
| to us, where formerly it was almost impossible to imagine 
that science, or even the practical affairs of earth, should 
in any way benefit. 
Having thus dealt very briefly with some of the more 
practical applications of the subject, I must now take you 
a somewhat distant journey to the sun and to the stars ; 
| and I must, in the first instance, attempt to connect the 
two perfectly distinct classes of phenomena which I have 
brought to your notice,—the phenomena, namely, of radi- 
ation, and the phenomena of absorption ; and this con 
HL 
Fic. 49.—Correspondence of some of the lines given out by iron vapour (below), and of some of the Fraunhofer lines in the solar spectrum. 
nection between radiation !and absorption is an instance 
of the slow growth of science. I remarked to you in the 
former lecture, that Fraunhofer, at the beginning of this 
century, had a very shrewd suspicion of the perfect co- 
incidence of place in the spectrum between certain dark 
lines which he saw in the spectrum of the sun, which I 
promised to explain to you on this occasion, and the 
bright lines in the spectrum of sodium. Yeu know how 
very simple the spectrum of sodium is : you will, perhaps, 
think it very strange indeed that such a simple thing was 
not explained very long ago. But Fraunhofer at the first 
suspected, and after him many of our greatest minds sus- 
pected, that there was some hidden, wondrously strange, 
connection between the double yellow line which you will 
remember is characteristic of sodium, and a certain double 
line which exists among the strange black lines of the 
solar spectrum, which I begged you to banish from your 
minds on the last occasion, when we were merely dealing 
But now I must ask you to bear with me 
concerning these black lines. I have been favoured by 
Dr. Gladstone with an extract from Dr. Brewster's note- 
book, dated St. Andrews, October 28,1841. Init Brewster 
says :—“I have this evening discovered the remarkable 
fact that, in the combustion of nitre upon charcoal, there 
are definite bright rays corresponding to the double lines 
of A and B, and the group of lines a in the space A B. 
The coincidence of two yellow rays with the two deficient 
ones at D, with the existence of definite bright rays in the 
nitre flame, not only at D but at A, a and 8, is so extra- 
| ordinary, that it indicates some regular connection between 
the two classes of phenomena.” The double lines A and 
B refer to some of these dark Fraunhofer lines in the solar 
spectrum, which for convenience of reference were at first 
called after the letters of the alphabet ; we now find that 
their number is so enormous that it is absolutely im- 
possible to attempt to grapple with them in any such 
method, but these names are still retained. 
The explanation of the coincidence between the two 
bright lines of burning sodium vapour and the two dark 
lines D in the solar spectrum was first given by Prof. 
Stokes about 1852. 
It is this. The light emitted by an incandescent 
vapour is due to the vibrations of its molecules, as a 
sound note emitted by a piano wire is due to the vibra- 
tion of the wire. You have only to go into a room where 
there is a piano, and sing a note, to find that the wire 
which corresponds to your note will respond to your 
voice. Now, in the same way, when light is passing 
through a vapour, the molecules of which vibrate at 
any particular rate, they will be urged into their own 
special rate of vibrations by the vibrations of the light 
which correspond to that particular rate waich is passing 
through them. Hence the light will, so to speak, be 
| sifted, and the force it has exercised in impelling the 
particles in the interrupting vapour to vibrate will tell 
upon it ; and in this way those particular vibrations which 
have had the work to do will be enfeebled. 
It is clear that the parts of the spectrum thus reduced 
in brilliancy will depend upon the vapour through which 
