204 NATURE [JuLy 27, 1899 
full of eagerness and penetration, enters Trinity | secretly tu find in the pure undulations the explanation of 
College, Cambridge; his name is Isaac Newton. | the beautiful phenomena he has reduced to such simple 
He has already in his village read Kepler's “Optics.” 
Almost immediately, and while following Barrow’s lectures 
upon optics, he studies the geometry of Descartes with 
passionate care; with his savings he buys a prism 
that he might examine the properties of colour and 
meditate deeply on the causes of gravitation. Eight 
years later his masters think him worthy to succeed 
Barrow in the Lucasian Professorship, and in his turn he 
also teaches optics. The pupil soon becomes greater than 
his teacher, and he gives out this great result : White 
light which seemed the type of pure light is not homo- 
geneous ; it consists of rays of different refrangibility, 
and he demonstrates it by the celebrated experiment of 
the solar spectrum, in which a ray of white light is de- 
composed into a series of coloured rays like a rainbow ; 
each shade of the colour is simple, for the prism does 
not decompose the shade. This is the origin of the 
spectral analysis. This analysis of white light brought 
Newton to explain the colours of the thin plates which 
are, for instance, observed in soap-bubbles. The funda- 
mental experiment, that of Newton’s rings, is one of the 
most instructive in optics, while the laws that govern it 
are of admirable simplicity. 
The theory was expounded in a discourse addressed 
to the Royal Society with the title, ““ A New Hypothesis 
concerning Light and Colour.” 
This discourse called forth from Hooke a sharp com- 
plaint. Hooke also had already examined the colour of 
thin plates, and endeavoured to explain them in the 
wave system. He had the merit, which Newton himself 
readily granted, to substitute for the progressive wave of 
Descartes a vibrating one—a new and extremely im- 
portant notion. He had even noticed the part of the 
two reflecting surfaces of the thin plate, and the mutual 
action of the reflected waves. Consequently Hooke 
should have been the very forerunner of the modern 
theory if he had had, as Newton, the clear intelligence of 
the simple rays. But his vague reasoning to explain the 
colours takes away all demonstrative value from his 
theory. 
Newton is very affected by this complaint of priority, 
and combats the arguments of his adversary by remark- 
ing that the wave theory is inadmissible because it does 
not explain the existence of the luminous ray and of the 
shadows. He denies the opinion that he has raised a 
theory ; he certifies that he does not admit either the 
wave hypothesis or the emission, but he says “ He shall 
sometimes, to avoid circumlocution and to represent it 
conveniently, speak of it as if he assumed it and pro- 
pounded it to be believed.” And, really, in the Proposi- 
tion XII. (second book of his Optics),! which constitutes 
what was since called the theory of fits, Newton remains 
absolutely on the ground of facts. He says simply, the 
phenomena of thin plates prove that the luminous ray 
is put alternatively in a certain state or fit of easy re- 
flexion and of easy transmission. He adds, however, 
that if an explanation of these alternative states is re- 
quired they can be attributed to the vibrations excited by 
the shock of the corpuscles, and propagated under the 
form of a wave in ether.” 
After all, notwithstanding his desire to remain on 
the firm ground of facts, Newton cannot help trying a 
rational explanation. He has too carefully read the 
writings of Descartes not to be heartily, as Huygens, a 
partisan of the universal mechanism and not to wish 
1 Prop. XII.—LHvery Ray of Light in its passage through any refracting 
Surface is put into a certain transient constitution or state, which in the 
progress of the Ray returns at equal Intervals, and disposes the Ray at every 
return to be easily transmitted through the next refracting Surface, and 
between the returns to be easily reflected by it. (Sir Isaac Newton, 
“ Opticks: or a Treatise of the Reflections, Refractions, Inflexions and 
plows of Light." London, 1718. Second edition, with additions. P. 293. 
~ Loc. cit., p. 
NO. 1552, VOL. 60] 
laws. But his third bock on optics more especially 
proves his Cartesian aspirations, and, above all, his per- 
plexity. His famous “ Queries” expose so forcibly his 
argument in favour of the wave theory of light that Thos. 
Young will later cite them as proof of the final conversion 
of Newton to the wave theory. Newton would certainly 
have yielded to this secret inclination had the inflexible 
logic of his mind allowed him to do so; but when after 
enumerating the arguments the wave theory of light offers 
in explanation of the intimate nature of light, when he 
arrived at the last “queries” he stops, as if seized by a 
sudden remorse, and throws it away. And the sole argu- 
ment is that he does not see the possibility of explaining 
the rectilinear transmission of light.!. Viewed from this 
standpoint the third book of Ofécks is no longer only an 
1 First, here is an extract from the ‘‘ Queries” which prove the leaning of 
Newton's views towards the undulatory theory and the Cartesian ideas. 
““ Query 12.—Do not the Rays of light in falling upon the bottom of the 
eye excite Vibrations in the 7znica Retina ? Which Vibrations, being pro- 
pagated along the Solid Fibres of the optick nerves into the brain, cause the 
Sense of seeing. . .” 
“ Query 13.—Do not several sorts of Rays make Vibrations of several big- 
nesses, which, according to their bignesses, excite sensations of several 
colours, much after the manner that the vibrations of the air, according to- 
their several bignesses, excite sensation of several sounds? And particularly 
do not the most refrangible rays excite the shortest vibrations for making a 
sensation of deep violet, the least refrangible the largest for making a 
sensation of deep red, &c.?...” 
“Query 18.— .. . Isnotthe heat of the warm room conyey’d through 
the vacuum by the vibrations of a much subtiler medium than air, which, 
after the air was drawn out remained in the vacuz? [ether] and is not this 
medium the same with that medium by which light is refracted and reflected, 
and by whose vibrations light communicates heat to bodies, and is put into« 
fits of easy reflection and easy transmission? ... And is not this medium 
exceedingly more rare and subtile than the air, and exceedingly more elastic 
and active? and doth it not readily pervade all bodies? and is it not (by its 
elastick force) expanded through all the heavens?” 
Newton, afterwards, considers the possible connection of this medium 
(ether) with the gravitation and the transmission of the sensations and 
motion in living creatures (queries 19 to 24). 
The dissymetrical properties of the two rays propagated in the Iceland 
spar, draw equally his attention (query 25 to 20). 
Here appears this sudden and unexpected going back. this sort of remorse 
from having too kindly expounded the resources of the Cartesian theory, 
based on the Ader22 ; he makes an apology as follows : 
“Query 27.—Are not all hypotheses erroneous which have hitherto been 
“Query 28.—Are not all hypotheses erroneous in which light is supposed 
to consist in pression or motion, propagated through a fluid medium?.. . 
and if it (light) consisted in pression or motion, propagated either in an 
instant or in time, it would bend into shadow. For pression or motion 
cannot be propagated in a fluid in right lines beyond an obstacle, which 
stops part of the motion, but will bend and spread every way into the 
quiescent medium which lies beyond the obstacle... . For a bell or a 
canon may be heard beyond a hill which intercept the light of sounding 
body, and sounds are propagated as readily through crooked pipes as 
through straight ones. But light is never known to follow crooked passages 
nor to bend into the shadow. . . .” 
Stopping before this objection Newton is forced to come back to the 
corpuscular theory. 
‘Query 29.—Are not the rays of light very small bodies emitted from 
shining substances? . . .” 
* Query 30.—Are not gross bodies and light convertible into one another 
...? The changing of bodies into light and light into bodies, is very con- 
formable to the course of nature, which seems delighted with transmuta- 
tlOUS.9% re: 
Logic urges him to go on with the old hypothesis of the vacwum and 
atoms, and even to invoke the authority of the Greek and Pheenician 
philosophers in this matter (query 28, p. 343), therefore it is not surprising 
to see his perplexity expressed by the following words :— 
“Query 31, and the last.—Have not the small particles of bodies certain 
powers, virtues, or forces, by which they act at a distance not only upon 
the rays of light for reflecting, refracting and inflecting them, but also upon 
one another for producing a great part of the phenomena of nature? .. .” 
But he perceives that he is going rather far, and compromising himself, 
therefore his secret tendency, developed in the foremost queries, reappear 
a little while :-— ‘ 
««. . . How these attractions may be performed I do not here consider. 
What I call attraction may be perform’d by impulse, or by some other 
means unknown tome...” 
Many other curious remarks could be made on the state of mind of the 
great physicist, geometer and philosopher, which is artlessly revealed in 
those ‘‘ queries.” The preceding short extracts are sufficient, I believe, to 
justify the conclusion which I get from the study of the 3rd Book, namely, 
that Newton had not at all on the mechanism of light the definite ideas 
which have been attributed to him as founder of the emission-theory. 
Really, he is hesitating between the two opposite systems, perceiving clearly 
their insufficiency ; and in this discussion he is endeavouring to go away as 
little as possible. from the facts. That is the reason for which he has 
stated no dogmatic theory. {[t would be, therefore, unjust to make Newton 
responsible for every consequence which the emission partisans have 
sheltered under his authority. 
