IVA Gah F 
361 
THURSDAY, AUGUST 20, 1885 
PROFESSOR STOKES ON LIGHT 
On Light as a Means 
(London: Mac- 
Burnett Lectures, Second Course. 
of Investigation. By G. G. Stokes. 
millan and Co., 18385.) 
HE interest raised by the first series of these lectures 
is fully sustained by this second instalment, though 
the subject-matter is of a very different order. 7/ev, the 
main question was the nature of light itself ; ow, we are 
led to deal chiefly with the uses of light as an instrument 
for indirect exploration. It is one of the most amazing 
results of modern science that the nature of mechanisms, 
too minute or too distant to be studied directly with the 
help of the microscope or the telescope, can be thus, in 
part at least, revealed to reason. This depends on the 
fact that a ray of light, like a human being, bears about 
with it indications alike of its origin and of its history ; 
and can be made to tell whence it sprang and through 
what vicissitudes it has passed. 
The lecturer begins by pointing out that this indirect 
use of light already forms an extensive subject ; and he 
then specially selects for discussion half-a-dozen important 
branches of it. Many readers will, we fear, be disap- 
pointed when they find that Disferszon (whether ordinary 
or anomalous) is not included in this list. It is tantalising 
to feel that we are not (for the present at least) to have 
the opinion of the author on the classical researches of 
Cauchy, or on the more recent speculations of Sellmeier, 
Helmholtz, and W. Thomson. It would, however, be 
unjustifiable to construe this omission into an indirect 
assertion that we do not yet know for certain what Dis- 
persion tells us :—though the parts of his wide subject 
which Prof. Stokes has selected for discussion are, each 
and all, such as give indications of a definitely interpret- 
able character. 
The first of these is Absorption. Here we have the 
explanation of the colours of bodies; the testing ray 
having gone in, and come out “shorn.” This leads to the 
application of the prism in the immediate discrimination 
of various solutions which, to the unaided eye, appear to 
have the same colour. It is shown how, by a mere glance, 
the chemist may often be saved from fruitless toil, occa- 
sionally from grave error. 
From the study of what rays are absorbed, the transition 
is an easy and natural one to the study of what becomes of 
them when they are absorbed. Here we have heating, 
chemical changes, phosphorescence, &c. The remainder 
of the lecture is devoted to an exceedingly interesting 
treatment of the beautiful subject of fluorescence. 
The second lecture begins with Rotation of the Plane 
of Polarisation of light by various liquids, with its im- 
portant application to saccharimetry. Then we have 
Faraday’s discovery of the corresponding phenomenon 
produced in the magnetic field, with its application in the 
discrimination of various classes of isomeric compounds. 
But the author, true to his system of mentioning practical 
applications only, omits all reference to quartz under the 
first of these heads and to gases under the second. And 
he does not even allude to the interesting questions 
recently raised as to the form of the general wave-surface 
in these curious circumstances. 
VOL. XXXII.—NO. 825 
Then comes the ‘‘still vexed” question of the history 
of Spectrum Analysis. The present view of it must, of 
course, be carefully read :—it is much too long to be here 
extracted in full, and to condense would be to mutilate it. 
Of course the claims of the author himself are the only 
ones to which scant justice is done. But the President 
of the Arztish Association of 1871 fortunately gave, in his 
opening address, the means of filling this /acwna. Just 
as the Gravitation-theory of an early Lucasian Professor 
was publicly taught in Edinburgh University before it 
became familiar among scientific men, so the present 
Lucasian Professor’s suggestions for the analysis of the 
solar atmosphere, by means of the dark lines in the spec- 
trum, were publicly explained in the University of 
Glasgow for ezght successive years before the subject 
became generally known through the prompt and wide- 
spread publicity given to the papers of Bunsen and 
Kirchhoff! The following are Sir William Thomson’s 
words of 1871 :—‘“‘It is much to be regretted that this 
great generalisation was not published to the world twenty 
years ago . . . because we might now be [szc] in pos- 
session of the inconceivable riches of astronomical results 
which we expect from the next ten years’ investigation by 
spectrum analysis, had Stokes given his theory to the 
world when it first occurred to him.” 
The third lecture is devoted to the information which 
spectrum analysis affords as to the chemical composition 
of the sun’s atmosphere, and its physical condition ; the 
classification of stars, the constitution of nebula, and the 
nature of comets. Those who still maintain that the tem- 
perature of the sun’s body is comparatively moderate are 
very summarily dealt with. Then follows a passage 
describing, in homely language fitted to be understanded 
of all, the state of the sun’s atmosphere. This is specially 
noteworthy, as showing how efficiently a Master can 
impress on his readers the most vivid ideas without 
requiring to use any but the simplest of language. 
The remarks on the nebulz and on comets will be read 
with great avidity ; and, by the majority of readers, with 
some surprise. For it is stated that the planetary nebule, 
“making abstraction of the stellar points, consist of 
glowing gas.” And of comets we find :—“ There can no 
longer be any doubt that the nucleus consists, in its inner 
portions at least, of vapour of some kind, and we must 
add incandescent vapour .. .” An ingenious suggestion 
as to the source of this incandescence is introduced as the 
“ creen-house theory.” The nucleus is supposed to be sur- 
rounded by an envelope of some kind, transparent to the 
higher but opaque to the lower forms of radiation. Thus 
solar heat can get freely at the nucleus, but cannot escape 
until it has raised the nucleus (in part at least) to incandes- 
cence. The coma and tail are formed by the condensation 
of small quantities of this vapour, so that they are mere 
mists of excessive tenuity. Herschel’s suggestion, that 
the development of the tail is due to electric repulsion 
exerted by a charge on the sun, is spoken of with ap- 
proval ; and the production of the requisite charge of 
the mist-particles is regarded as a concomitant of con- 
densation. Nothing, however, is said as to the opposite 
charge which the comet itself must receive, nor of the 
peculiar effects which would arise from this cause :— 
whether in the form of a modification of the shape of the 
comet’s head, or of a modification of its orbit and period 
R 
