896 
principle that the impression of a wave of light 
on a screen is to be found by considering the simul- 
taneous effect of the wavelets propagated from all 
the points disturbed at any previous moment (which 
points form what is called the front of a wave). 
It is only when these partial impressions concur 
that they are strong enough to affect the sense of 
vision ;—(2), the representation of the phenomenon of 
double refraction by a wave of two sheets, or the 
simultaneous propagation of spherical and spheroidal 
waves in one and the same medium. Of the last of 
these doctrines I shall speak in the next section in 
connection with the discoveries of Malus. With re- 
spect to the former I may here observe that it gives 
the only satisfactory explanation of the primary diffi- 
MATHEMATICAL AND PHYSICAL SCIENCE. 
[Diss. VL. 
mise, and must be held, we think, to have left behind 
him no substantive theory of light worthy of the 
name. The question never perhaps very seriously 
engaged his attention after the publication of Huy- 
gens’ book ;? and we know that about that time his 
intellectual energies received a shock which left him 
indisposed for the fatigue of constructing new theories, 
and still more disinclined to publish them. 
Euler, though he professed to defend an undula- 
(457.) 
tory theory of light, treated the subject.in a point of Euler. 
view chiefly mathematical, and betrayed that uncon- 
cern about physical theories which characterized a 
mind steeped in geometric abstractions. He even 
retrograded ; for he did not maintain the Huygenian 
explanation of the cause of detinite shadows. 
culty of the undulatory hypothesis,—namely, that a 
beam of light admitted by a hole in a screen pursues 
a rectilincar course afterwards, instead of spreading 
sideways, as do waves in water, and waves of sound. 
Young on the Undulatory Theory—Difrraction, _(458.) 
—One hundred and ten years after the publication of os a 
the Trraité de la Lumiére of Huygens, Dr Thomas tory thet 
Young re-opened the theory of light or “‘ Physical Op- —Diffrac- 
Huygens shows, on elementary and convincing prin- 
ciples, that the lateral impressions of the wave are 
rapidly extinguished by the want of concurrence of 
the impulses which they communicate to the ether. 
This is necessarily true when the breadth of the aper- 
ture is such as to exceed vastly the length of a wave ; 
and such is always the case with light, but rarely in 
any other instance. It is, in short, only imme- 
diately in front of the aperture that the disturbances 
originating in every part of the front of the original 
wave embraced within the aperture, concur in pro- 
ducing an accordant movement on the ether. 
tics,” as he termed it. His experiments and reasonings *- 
will be found in a series of papers in the Philoso- 
phical Transactions for 1800, 1801, 1802, and 1803.° 
These memoirs are of no great length, and deserve 
the most careful study. They are perhaps among the 
clearest and plainest of Young’s writings, although 
blamed at the time for defects precisely the reverse. 
They are eminently marked by penetration, profound 
induction, and candour of argument. Starting from 
his studies in acoustics, the transition to optical 
questions is extremely gradual. Young was, cha- 
racteristically, a good musician in practice, as well as 
(455.) This principle, more fully stated, by which every a profound one in theory, and his paper of 1799 is 
The prin- ]yminiferous disturbance of the ether is considered principally acoustical. In it he attaches conse- 
pd as the resultant of all the pre-existing disturbances quence to showing that the divergence of sound from 
to which it is due, constitutes what is sometimes the direction of its emission is deke and less com~ 
called the principle of Huygens, of which I shall pletethan it is commonly believed to be, and he applies 
have more to say hereafter. theanalogy to the existence of rays of light and definite 
(456.) Neither at the time of its publication, nor for shadows. In one short section he sums up the chief 
Newton’s more than a century afterwards, was the value of points of optical doctrine which lead him to prefer 
thonsture these reasonings understood. It would be beside the theory of Huygens to that of Newton. Amongst 
of light. our present object to discuss Newton’s opinions; but the facts better explained by waves than corpus- 
it is too certain that he did not allow Huygens’ argu- 
ments on the undulatory nature of light to have any 
weight with him. Not that he was averse (as is 
often supposed) to the presence of Ether as modify- 
ing the corpuscular theory of light; on the con- 
trary, in many of his minor writings he speaks of 
its existence as all but certain, and as a requisite 
adjunct to the corpuscular hypothesis to which he 
had been led by the facts of reflection and refrac- 
tion. But he never adjusted the terms of a compro- 
cles, we find reckoned Inflection and the Colours of 
thin plates. But all this is stated in a very general 
way, evidently rather as a conclusion towards which 
his mind had for some time been tending, than as the 
result of demonstrative proofs. In his paper of 1801 
the undulatory doctrine is methodically expounded in 
aseries of propositions, accompanied by proofs. The 
accurate definition of shadows is shown to be possible 
and natural on that theory, as well as the usual phe- 
nomena of reflection, refraction, and total reflection. 
1 Thus Newton writes in 1675 :—“ Were I to assume an hypothesis, it should be this, if propounded more generally, so as not 
to determine what light is farther than that it is something or other capable of exciting vibrations in the ether ; for thus it will 
become so general and comprehensive of other hypotheses as to leave little room for new ones to be invented.” And again,— 
“Do not the most refrangible rays excite the shortest vibrations [of the retina], the least refrangible the largest ?”—Birch’s Hist. 
of the Royal Society, quoted in Young’s Lectures, ii. 615,617. Sir D. Brewster (Life of Newton, 1855, vol. i. p. 148) considers 
that some passages in the later editions of Newton’s Optics show that he had departed from any theory of undulations. 
? The Optics, though published in 1704, had been written principally in 1675 and 1687 (see Preface). ‘ 
% They may be more conveniently consulted as reprinted in the second volume of his Lectures on Natwral Philosophy, and in 
his Miscellaneous Works, vol. i. 
