162 
SOLAR PHYSICS > 
Its 
HAVE to address you in this course of lectures on what we 
know of the infra-red end of the spectrum and its relation 
to solar physics. I will commence by asking a question, and 
endeavour to answer it in such a way as will, I hope, be under- 
stood. The question I propound is, How do we know that there 
are any rays below the red rays of the spectrum? Jn answering 
the question I would beg you to remember that every body in 
motion possesses what we call energy, or a capacity for doing 
work, be the motion a wave motion or a direct motion. Let us 
take one or two examples of waves : first, that of water, which is 
familiar to us, I need scarcely point out that a wave of the sea 
is capable of doing an immense amount of work, not to say 
mischief ; there is no doubt, then, that it is capable of doing 
work, and this we may take as the true definition of energy, 
existing in a body, viz. the capacity of doing work. Whence, then, 
does a wave derive its energy? Perhaps we may have to travel 
many miles from the place where we find our wave, Travelling 
to the origin of the waves, we shall no doubt find that a wind 
has generated them, and in reality it is the energy possessed by 
the wind which is carried by the waves to the distant shore. The 
energy possessed by the wind has not been directly expended on 
our coast, but when transmitted by the waves this same energy 
is applied in different manner, and by this difference in appli- 
cation it becomes effective. We all know, for instance, that a 
child may ring a church bell if he give a pull at the right inter- 
vals of time, and so, by timing the impact of waves correctly, it is 
possible for them to do work which in any other way would be 
impossible, Another example of the energy of waves is the tuning- 
fork, as in the experiment which Mr. Lockyer showed you. You 
will recollect that he demonstrated that if one tuning-fork was 
brought near another of the same pitch the second took up the 
vibration of the air. The tuning-fork which was struck, or 
bowed, generated waves in the air carrying some part of the 
energy of the vibrating prongs to be expended on the second 
tuning-fork, and as this tuning-fork vibrated in the same period 
as the first one, each blow of the air-waves was essentially well- 
timed, and the fork was thus set in motion. You will also recol- 
lect that a a fork not of the same pitch—that is, not sounding 
the same note—was unable to cause vibration in the second 
fork; and this was simply because the energy was applied at 
wrong intervals of time. In the case of the tuning-fork, then, 
the air is the medium through which this energy was conveyed. 
With light we have the same kind of motion in the luminiferous 
ether: the motions of the molecules swinging in the source of 
light may, for the sake of illustration, be looked upon as com- 
posed of an infinite number of tuning-forks, the ether, instead of | 
the air, carrying their energy in all directions. How can the 
energy in the ether show itself? In the first place it must meet 
with some obstruction, and secondly that obstruction must be 
capable of vibrating with it, and thus damp or destroy the waves. 
The destruction of the wave motion in the ether is known as the 
absorption, and thus we see that where there is absorption there 
work of some kind must be done. ‘the work, then, that light can 
perform is this. [When I say light, I say it with a definite object. 
{ft has been said that it is nonsense to talk about dark light ; but 
it is no more nonsense to talk about dark light than to talk of a 
white violet, a yellow rose, and so on. Therefore, I prefer to 
call the whole ether vibrations with which we are acquainted, 
light, until we get a more authoritative definition.] The work 
that light may perform then is this, it may cause certain appli- 
ances in our eye to vibrate (and perhaps also cause chemical de- 
composition on the colouring matter of some membrane which is 
placed near the retina), which gives us the sensation of vision. 
Secondly, it may cause the molecules of the material body on 
which it falls to vibrate more freely than they do when in a 
normal state of vibration, and thus raise the temperature of the 
body, (It must be recollected that physicists suppose the mole- 
cules of all matters to be in active vibration, and a rise of tem- 
perature simply means an increase of those molecular motions). 
Inthe third place it may cause the atoms which compose the 
molecules to vibrate more energetically than they do under 
ordinary circumstances, and cause one or more of the atoms to 
swing off, as it were, and thus create a new molecule ; in other 
words, cause a dissociation of the molecule. We may sum up 
our definition by saying that the presence of light can be known 
* Lecture delivered on May 25, 1881, at the Lecture Theatre, South Ken- 
WATURE 
sington Museum, by Capt. Abney, R.E., F.R.S. 
[Dec. 15, 1881 
by three distinct kinds of work. It may be known by its 
causing the sensation of vision; it may be known by arise in 
temperature of the body on which it falls; and it may be also 
known by the chemical action which it induces. I think, 
then, we have an answer to the question which I propounded, 
How can we tell that there are rays which exist below the 
visible red of the spectrum? If they exist, they must be shown 
by a rise in the temperature of any body which may absorb those 
rays when placed in their path, or by their chemical effect. That 
they do not give rise to the sensation of vision I need scarcely 
say. 
The dark rays were discovered in the years 1800 and 1801 by 
Sir William Herschel, who was investigating the solar surface 
with a telescope. Finding that the heat sent to the eye was 
unbearable, he wished to obtain some medium to cut off those par- 
ticular rays which gave the heating effect. In order to do that 
he undertook a series of investigations of the spectrum, in what we 
should now call perhaps a rough kind of way, ina manner which 
I will show you on the screen, A beam of light was passed 
through a prism fixed horizontally against a slit in a wall, being 
bent so that the spectrum fell upon a table beneath, on which he 
ruled lines marking the boundaries of the colours. On a sloping 
board turning on castors he placed three thermometers in a line, 
two of which he caused to lie within the spectrum, the third 
remaining outside it. He then noted the height of the mercury 
in all three of the thermometers, and thus compared the two in 
the spectrum with that lying beyond it (I may say that the dia- 
meters of the bulbs of the two thermoineters in the spectrum, 
which is rather an important point, were one-eighth of an inch 
and half an inch respectively), Not only did Sir William Herschel 
use thermometers, but he also used the principle of absorption 
to increase their indications, for he blackened those thermometers 
with China black, He found he got a greater effect by using 
lampblack than by u-ing the bare bulbs of the thermo- 
meters. He commenced by placing his two thermometers 
in the violet, and he found he got a certain rise of mercury. 
Having made a scale in accordance with the ruled lines on his 
table, he set up at the point indicating the violet an ordinate 
also to scale, showing the number of degrees of rise in the 
thermometer at that particular point. Then in the indigo he 
set up another ordinate indicating the degrees of rise there, 
and so on at all these different points ; so that he was able to 
construct, as it were, a mountain of the heat effect due to the 
spectrum in all parts (Fig. 1). Having gone in this way over the 
} : i ; 
TOP RED. . ORWCE YELLOW | GREEN | BLUE }!N2Sncter 
Fic. «. 
whole of the visible solar spectrum, he found there was a rise in the 
two thermometers, as he approached the red from the violet, (It 
must be recollected that before his time there was no knowledge of 
any rays which existed below the red), He therefore ruled lines 
on his table beyond the red, and having reached the limit of the 
luminous spec'rum, he shifted his thermometers beyond, and 
found that they rose even higher than in the red. This led him 
to continue the experiment, and he found by going a long way 
beyond the red he still got a slight trace of rise in the mercury of 
his thermometers. By this means he was able to construct his 
well-known curve (which answers to a curve of energy) in the 
very simple manner shown in Fig, tr, I shall have to refer to this 
curve in another lecture, and I want you to fully bear in mind that 
the heights of every part of this curve answered to a comparative 
measure of the energy of the particular parts of the spectrum — 
