8 REPORT—1874. 
might be, and would be, perfectly content to fight the battle of mathematical physics 
on the ground which Comte himself has chosen. We have put one important 
question to the history of science, let us put another. 
Has the effect of theories of light upon the progress of real optical knowledge 
(knowledge which Comte himself would admit to be real) been beneficial or 
injurious ? 
This question belongs to a class to which the answer is never easy. It is never 
an easy task to abstract one from a group of causes which concur in the production 
of an effect, and then determine how the effect would have been changed by such 
removal, Still we may succeed in obtaining at least a partial answer to the 
question. 
It has been frequently remarked as one of the benefits conferred upon physical 
science by theory, that it suggests experiment. Nowhere is this principle more 
strongly exemplified than in the history of perhaps the greatest name in optical 
science—I mean, Fresnel. He is an experimentalist certainly ; but he is an experi- 
mentalist because he is atheorist. His most valuable experiments had their origin 
in the desire to test the truth of a theory. The experiment with the two mirrors 
were devised to test Young’s principle ofinterference. His diffraction experiments 
were devised at first to test the truth of Young’s theory ; and when that had been 
found to be inconsistent with fact, then to test the truth of his own. .And, not to 
multiply instances, the experiments by which he established the existence of cir- 
cular polarization, and ascertained the true nature of the light which passes along 
the axis of a quartz crystal, were suggested by theory. 
Among the motives which induced Jamin to undertake the experimental 
researches which have given to science such valuable results, not the least was the 
desire to test the truth of an hypothetical principle of Fresnel and of a theoretic 
formula of Cauchy. And quite recently M. Abria has made an elaborate examina- 
tion of uniaxal refraction for the purpose of testing the truth of the construction 
of Huyghens. I may here sero that it is much to be desired that some com- 
petent observer would undertake the yet more difficult task of verifying experi- 
mentally the wave-surface of Fresnel. 
But to revert to the general subject. If any physicist is inclined to agree with 
the views of Comte upon this subject, let me propose to him the following test :— 
Let him strike out of physical optics every thing which that science owes to theories 
of light, and then let him try to write a treatise on the subject, excluding the lan- 
guage and the ideas of theory. Finally, let him compare his work with some trea- 
tise in which these aids have not been neglected, and judge himself of their relative 
value. Theoretic science need not be afraid of the result. 
Naturally suggested by the subject which we have been considering, namely, 
the tendency of scientific progress to a reduction of all physical science under the 
power of mathematical analysis, is the gradual development of connexions between 
the different members of that great group to which we give the name of physical 
science. And among the instances of such growing relationship I take, also sug- 
gested by the topics which have engaged us, the connexion between optics and 
chemistry. I only say “suggested ” by our former subject, for I do not desire to 
attach any undue significance to the fact that of these connected sciences one may 
already be called a mathematical science. As yet the connexion between these 
sciences has consisted principally in the introduction into chemistry of an analysis 
in some respects more refined than any which has been hitherto known. And this 
fact does not in itself indicate the extension to chemistry of the mathematical 
character which belongs to physical optics. Still, if we hold the assumption 
of this character by any science to be the mark of perfection, we shall be in- 
clined to regard every improvement in its instruments of research as tending in 
that direction. 
In speaking of the connexion between Optics and Chemistry, the topic which 
will occur first to every one is the Spectroscope; but on this part of the subject it 
is not necessary that I should dwell. It has so largely occupied the attention of 
physicists, and has been so fully treated by those who have made it their special 
study, that I could not hope to add any thing to what they have said. I would 
only observe that the spectroscope has enabled chemistry to overleap a barrier 
