(450.) 
(451.) 
(452.) 
The undu- 
latory 
theory of 
light— 
Hooke. 
Cuar. V, § 1] 
Many of the figures, though of hackneyed subjects, are 
represented in a novel manner; there seems not a 
line drawn at random. Such figures often illustrate 
better than pages of description, the clearness with 
which an author has conceived to himself the neces- 
sary results of his own principles. An example of 
this may be found in Plate XXX., fig. 442 of the 
first volume, which, as Arago relates, served to de- 
monstrate to him, when he visited Dr Young in 
1816, that he and Fresnel had been anticipated on 
one point more than they believed." 
The absence of algebraic formule from this work 
was as characteristic of Dr Young as their copious 
introduction into articles which he subsequently con- 
tributed to the Quarterly Review. He had decided 
upon writing a book without symbols, and he wrote 
it, though it gave additional trouble both to himself 
and the reader. 
We shall now proceed to trace the progress of the 
undulatory theory of light, the greatest physico- 
mathematical discovery of our time, in the establish- 
ment of which Young acted the leading part. 
Undulations of Light—Hooke and Huygens.— 
The idea of accounting for the effects and modi- 
fications of luminous impressions by disturbances 
propagated through a very elastic medium was by 
no means new at the commencement of the present 
century. We do not, indeed, attach much impor- 
tance to the so-called anticipations of Grimaldi and 
Hooke in the seventeenth century. The former, 
amongst his valuable experiments on the deflection of 
light and fringes of shadows, had used an expression 
as to illumination being diminished by the addition of 
light, which is true in fact, and is a correct deduction 
from the law-of interference as we now understand it, 
Hooke, in his Micrographia, asserts light to consist 
in “ quick, short, vibrating motion ;” but his expla- 
nation of refraction by it is altogether erroneous ; 
and his application of it to the doctrine of the colours 
of thin plates, though admitted by the candid Young 
to be an anticipation (unknown to him at the time) 
of his own, has in it no more than a germ of truth 
(like so many of Hooke’s ingenious hints, afterwards 
claimed by him as discoveries), which yet only ex- 
plains the fact on which it is founded by means of 
an additional and gratuitous assumption. The germ 
of truth in Hooke’s writings is this, that the colours 
in question depend upon a mixture of the light re- 
flected at the first surface of the thin plate with “a 
kind of fainter ray”? propagated from the second sur- 
face backwards: the gratuitous assumption is, that 
“ this compound or duplicated pulse does produce on 
the retina the sensation of a yellow ;” why it does so 
OPTICS.— YOUNG. 
895 
is not explained. This was in 1664, before even New- 
ton was acquainted with the analysis of white light. 
Hooke’s idea that yellow, or any other colour, was 
the result of the conflict of pulses simultaneously 
reaching the eye, was an assertion, admissible, per- 
haps, at that time, as expressing a fact; but surely 
not a proof of interference producing reinforcement 
or annihilation of light, as taught by Young. I am 
not aware that Hooke ever even reiterated his opinions 
on this subject after Newton had analysed the phe- 
nomena experimentally, and shown that the colours 
of thin plates result from the superposition of bright 
and dark rings of different prismatic hues, each with 
its appropriate diameter. It was then apparent that 
colour was only an indirect effect of interference. 
But whatever may be thought of the theories of (453.) 
Hooke, those of Huygens deserve a far more eminent Huygens’ 
placein history. Having already been succinctly ad- 
verted to in Professor Playfair’s dissertation, we will 
only observe that the T'raité de la Lumiére (1690) is 
an admirably composed and reasoned treatise on the 
phenomena of light on the undulatory hypothesis. 
The uniformity of its propagation through the celes- 
tial spaces, its rectilinear course in ordinary circum- 
stances, the laws of its reflection and refraction. are 
there explained with a degree of elegance and preci- 
sion which ought to have excited (we must think) 
general attention and assent, but for the ascendancy 
of Newton’s authority, and the astonishing and beau- 
tiful nature of the experiments on which his theories 
were based; whereas Huygens referred to few expe- 
riments except those of the simplest kind, and the 
phenomena of colour were (for good reasons) left 
chiefly out of view.? Such being the case, Huygens 
may fairly be considered as the author of the undu- 
latory theory, which he supported by such convincing 
proofs, 
The fundamental principle of the Huygenian 
doctrine was the same as that which Hooke ad- Explana- 
mitted, which was probably far older than his time, 
namely, that all space, including the interior of 
transparent bodies, contains an ether whose pulsa- fraction. 
tions communicate the sense of light to the eye, as 
waves in air convey to the ear impressions of sound. 
To this he added the assumption, that in refracting 
media, such as glass, the pulsations are retarded ; 
whereas in Newton's theory, as is well known, the 
propagation of light is assumed to be fastest in dense 
media, The “law of the sines” in refraction, is de- 
duced as a consequence ; and one of the prettiest ap- 
plications made of it is to the phenomena of atmo- 
spherical refraction. But the most important features 
of the whole investigation are these two—(1), the 
1 The reference of Arago, in his original éloge of Young, is to p. 387 of the Lectures, obviously by mistake. In the first 
volume of his (Arago’s) collected works it is corrected into p. 787, which is as certainly correct. I have supposed fig. 442 to be 
more probably the one referred to than 445, to which Dr Peacock refers (Life of Young, p. 389). 
® Nevertheless the Huygenian Theory of Light was propounded as the subject of a thesis at St Andrews by David Gregory, 
whilst professor of mathematics there, within about a year of its publication (including also the Newtonian doctrine of gravity), 
—a pleasing proof of the activity which then reigned*in that university. Principal Lee possesses the original programme. 
