_The Sensation of different Colours depends on the different frequency
of Vibrations, excited by Light in the Retina._
_Passages from_ NEWTON.
“The objector’s hypothesis, as to the fundamental part of it, is not
against me. That fundamental supposition is, that the parts of bodies,
when briskly agitated, do excite vibrations in the ether, which are
propagated every way from those bodies in straight lines, and cause a
sensation of light by beating and dashing against the bottom of the
eye, something after the manner that vibrations in the air cause a
sensation of sound by beating against the organs of hearing. Now, the
most free and natural application of this hypothesis to the solution
of phenomena, I take to be this: that the agitated parts of bodies,
according to their several sizes, figures, and motions, do excite
vibrations in the ether of various depths or bignesses, which, being
promiscuously propagated through that medium to our eyes, effect in us
a sensation of light of a white colour; but if by any means those of
unequal bignesses be separated from one another, the largest beget a
sensation of a red colour, the least or shortest of a deep violet, and
the intermediate ones of intermediate colours; much after the manner
that bodies, according to their several sizes, shapes, and motions,
excite vibrations in the air of various bignesses, which, according
to those bignesses, make several tones in sound: that the largest
vibrations are best able to overcome the resistance of a refracting
superficies, and so break through it with least refraction; whence the
vibrations of several bignesses, that is, the rays of several colours,
which are blended together in light, must be parted from one another by
refraction, and so cause the phenomena of prisms, and other refracting
substances; and that it depends on the thickness of a thin transparent
plate or bubble, whether a vibration shall be reflected at its further
superficies, or transmitted; so that, according to the number of
vibrations, interceding the two superficies, they may be reflected or
transmitted for many successive thicknesses. And, since the vibrations
which make blue and violet, are supposed shorter than those which make
red and yellow, they must be reflected at a less thickness of the
plate: which is sufficient to explicate all the ordinary phenomena of
those plates or bubbles, and also of all natural bodies, whose parts
are like so many fragments of such plates. These seem to be the most
plain, genuine, and necessary conditions of this hypothesis. And they
agree so justly with my theory, that if the animadversor think fit to
apply them, he need not, on that account, apprehend a divorce from it.
But yet, how he will defend it from other difficulties, I know not.”
(Phil. Trans. Vol. VII. p. 5088. Abr. Vol. I. p. 145. Nov. 1672.)
“To explain colours, I suppose, that as bodies of various sizes,
densities, or sensations, do by percussion or other action excite
sounds of various tones, and consequently vibrations in the air
of different bigness; so the rays of light, by impinging on the
stiff refracting superficies, excite vibrations in the ether,—of
various bigness; the biggest, strongest, or most potent rays, the
largest vibrations; and others shorter, according to their bigness,
strength, or power: and therefore the ends of the capillamenta of the
optic nerve, which pave or face the retina, being such refracting
superficies, when the rays impinge upon them, they must there excite
these vibrations, which vibrations (like those of sound in a trunk or
trumpet) will run along the aqueous pores or crystalline pith of the
capillamenta, through the optic nerves, into the sensorium;—and there,
I suppose, affect the sense with various colours, according to their
bigness and mixture; the biggest with the strongest colours, reds and
yellows; the least with the weakest, blues and violets; the middle
with green; and a confusion of all with white, much after the manner
that, in the sense of hearing, nature makes use of aerial vibrations of
several bignesses, to generate sounds of divers tones; for the analogy
of nature is to be observed.” (BIRCH Vol. III. p. 262. Dec. 1675.)
“Considering the lastingness of the motions excited in the bottom of
the eye by light, are they not of a vibrating nature?—Do not the most
refrangible rays excite the shortest vibrations,—the least refrangible
the largest? May not the harmony and discord of colours arise from the
proportions of the vibrations propagated through the fibres of the
optic nerve into the brain, as the harmony and discord of sounds arise
from the proportions of the vibrations of the air?” (Optics, Qu. 16,
13, 14.)
_Scholium_. Since, for the reason here assigned by NEWTON, it is
probable that the motion of the retina is rather of a vibratory than of
an undulatory nature, the frequency of the vibrations must be dependent
on the constitution of this substance. Now, as it is almost impossible
to conceive each sensitive point of the retina to contain an infinite
number of particles, each capable of vibrating in perfect unison with
every possible undulation, it becomes necessary to suppose the number
limited, for instance, to the three principal colours, red, yellow,
and blue, of which the undulations are related in magnitude nearly as
the numbers 8, 7, and 6; and that each of the particles is capable of
being put in motion less or more forcibly, by undulations differing
less or more from a perfect unison; for instance, the undulations of
green light being nearly in the ratio of 6-1/2, will affect equally the
particles in unison with yellow and blue, and produce the same effect
as a light composed of those two species: and each sensitive filament
of the nerve may consist of three portions, one for each principal
colour. Allowing this statement, it appears that any attempt to produce
a musical effect from colours, must be unsuccessful, or at least that
nothing more than a very simple melody could be imitated by them; for
the period, which in fact constitutes the harmony of any concord, being
a multiple of the periods of the single undulations, would in this case
be wholly without the limits of sympathy of the retina, and would lose
its effect; in the same manner as the harmony of a third or a fourth is
destroyed, by depressing it to the lowest notes of the audible scale.
In hearing, there seems to be no permanent vibration of any part of the
organ.