ニュートンのプリズム分光実験が1666年である根拠

アイザック・ニュートンがプリズムを使って太陽光のスペクトルを観察する実験を行ったのは１666年と言われています。たくさんの本に「1666年」と記載されていますが、ニュートンの「光学」が刊行されたのは1704年です。１666年の根拠は何か調べてみました。

ニュートンは1665年に万有引力を発見していますが、この頃、ロンドンではペスト菌が大流行しており、ケンブリッジ大学が閉鎖となりました。ニュートンは大学の雑務から解放され、しばらくの間、故郷に帰りました。ニュートンは、かねてから考えていたことを、ゆっくりと研究することができる時間を得て、微分積分学、プリズム分光、万有引力の研究を行いました。

その後、ニュートンは大学に戻り、1669年にケンブリッジ大学の数学の教授職であるルーカス教授となりました。教授となっての最初の功績は、数学ではなく、反射式望遠鏡の発明でした。王位教会はこの反射式望遠鏡に注目し、1671年にニュートンに反射式望遠鏡を提出するよう要求しました。ニュートンは反射式望遠鏡の改良型を作成し、王位教会に提出、多くの専門家から賞賛されました。

ニュートンは、反射望遠鏡の発明の経緯について、王立協会宛に1672年2月6日付けで「New Theory About Light and Colour（光と色の関する新理論）」と題した手紙を送りました。

この手紙の冒頭に、1666年の初めにプリズム実験をしたことが書かれているのです。

New Theory About Light and Colour
by Isaac Newton

Sir,

To perform my late promise to you, I shall without further ceremony acquaint you that in the beginning of the year 1666 (at which time I applied myself to the grinding of optic glasses of other figures than spherical) I procured me a triangular glass prism to try therewith the celebrated phenomena of colours. And in order thereto having darkened my chamber and made a small hole in my window-shuts to let in a convenient quantity of the sun's light, I placed my prism at his entrance that it might be thereby refracted to the opposite wall. It was at first a very pleasing divertissement to view the vivid and intense colours produced thereby; but after a while, applying myself to consider them more circumspectly, I became surprised to see them in an oblong form, which according to the received laws of refraction I expected should have been circular.

They were terminated at the sides with straight lines, but at the ends the decay of light was so gradual that it was difficult to determine justly what was their figure; yet they seemed semicircular.

Comparing the length of this coloured spectrum with its breadth, I found it about five times greater, a disproportion so extravagant that it excited me to a more than ordinary curiosity of examining from whence it might proceed. I could scarce think that the various thickness of the glass or the termination with shadow or darkness could have any influence on light to produce such an effect; yet I thought it not amiss first to examine those circumstances, and so tried what would happen by transmitting light through parts of the glass of divers thicknesses, or through holes in the window of divers bignesses, or by setting the prism without, so that the light might pass through it and be refracted before it was terminated by the hole. But I found none of those circumstances material. The fashion of the colours was in all, these cases the same.

Then I suspected whether by any unevenness in the glass or other contingent irregularity these colours might be thus dilated. And to try this, I took another prism like the former and so placed it that the light, passing through them both, might be refracted contrary ways, and so by the latter returned into that course from which the former had diverted it. For by this means I thought the regular effects of the first prism would be destroyed by the second prism but the irregular ones more augmented by the multiplicity of refractions. The event was that the light which by the first prism was diffused into an oblong form was by the second reduced into an orbicular one with as much regularity as when it did not at all pass through them. So that, whatever was the cause of that length, 'twas not any contingent irregularity.

The gradual removal of these suspicions at length led me to the experimentum crucis, which was this; I took two boards, and placed one of them close behind the prism at the window, so that the light might pass through a small hole made in it for the purpose and fall on the other board, which I placed at about 12 feet distance, having first made a small hole in it also, for some of that incident light to pass through. Then I placed another prism behind this second board so that the light, targeted through both the boards, might pass through that also, and be again refracted before it arrived at the wall. This done, I took the first prism in my hand, and turned it to and fro slowly about its axis, so much as to make the several parts of the image cast on the second board successively pass through the hole in it, that I might observe to what places on the wall the second prism would refract them. And I saw by the variation of those places that the light tending to that end of the image towards which the refraction of the first prism was made did in the second prism suffer a refraction considerably greater than the light tending to the other end. And so the true cause of the length of that image was detected to be no other than that light consists of rays differently refrangible, which, without any respect to a difference in their incidence, were, according to their degrees of refrangibility, transmitted towards divers parts of the wall.

I shall now proceed to acquaint you with another more notable difformity in its rays, wherein the origin of colours is unfolded: concerning which I shall lay down the doctrine first and then for its examination give you an instance or two of the experiments, as a specimen of the rest.

The doctrine you will find comprehended and illustrated in the following propositions.

1. As the rays of light differ in degrees of refrangibility, so they also differ in their disposition to exhibit this or that particular colour. Colours are not qualifications of light, derived from refractions or reflections of natural bodies (as 'tis generally believed), but original and connate properties which in divers rays are divers. Some rays are disposed to exhibit a red colour and no other, some a yellow and no other, some a green and no other, and so of the rest. Nor are there only rays proper and particular to the more eminent colours, but even to all their intermediate gradations.

2. To the same degree of refrangibility ever belongs the same colour, and to the same colour ever belongs the same degree of refrangibility. The least refrangible rays are all disposed to exhibit a red colour, and contrarily those rays which are disposed to exhibit a red colour are all the least refrangible. So the most refrangible rays are all disposed to exhibit a deep violet colour, and contrarily those which are apt to exhibit such a violet colour are all the most refrangible. And so to all the intermediate colours in a continued series belong intermediate degrees of refrangibility. And this analogy 'twixt colours and refrangibility is very precise and strict; the rays always either exactly agreeing in both or proportionally disagreeing in both.

3. The species of colour and degree of refrangibility proper to any particular sort of rays is not mutable by refraction nor by reflection from natural bodies nor by any other cause that I could yet observe. When any one sort of rays hath been well parted from those of other kinds, it hath afterwards obstinately retained its colour, notwithstanding my utmost endeavours to change it. I have refracted it with prisms and reflected it with bodies which in daylight were of other colours; I have intercepted it with the coloured film of air interceding two compressed plates of glass; transmitted it through coloured mediums and through mediums irradiated with other sorts of rays, and diversely terminated it; and, yet could never produce any new colour out of it. It would by contracting or dilating become more brisk or faint and by the loss of many rays in some cases very obscure and dark; but I could never see it changed in specie.

4. Yet seeming transmutations of colours may be made, where there is any mixture of divers sorts of rays. For in such mixtures, the component colours appear not, but by their mutual allaying each other constitute a middling colour. And therefore if by refraction or any other of the aforesaid causes the difform rays latent in such a mixture be separated, there shall emerge colours different from the colour of the composition. Which colours are not new generated, but only made apparent by being parted; for if they be again entirely mixed and blended together, they will again compose that colour which they did before separation. And for the same reason, transmutations made by the convening of divers colours are not real; for when the difform rays are again severed, they will exhibit the very same colours which they did before they entered the composition—as you see blue and yellow powders when finely mixed appear to the naked eye green, and yet the colours of the component corpuscles are not thereby transmuted, but only blended. For, when viewed with a good microscope, they still appear blue and yellow interspersedly.

5. There are therefore two sorts of colours: the one original and simple, the other compounded of these. The original or primary colours are red, yellow, green, blue, and a violet-purple, together with orange, indigo, and an indefinite variety of intermediate graduations.

6. The same colours in specie with these primary ones may be also produced by composition. For a mixture of yellow and blue makes green; of red and yellow makes orange; of orange and yellowish green makes yellow. And in general if any two colours be mixed which, in the series of those generated by the prism, are not too far distant one from another, they by their mutual alloy compound that colour which in the said series appeareth in the mid-way between them. But those which are situated at too great a distance, do not so. Orange and indigo produce not the intermediate green, nor scarlet and green the intermediate yellow.

7. But the most surprising and wonderful composition was that of whiteness. There is no one sort of rays which alone can exhibit this. 'Tis ever compounded, and to its composition are requisite all the aforesaid primary colours, mixed in a due proportion. I have often with admiration beheld that, all the colours of the prism being made to converge and thereby to be again mixed as they were in the light before it was incident upon the, prism, reproduced light, entirely and perfectly white, and not at all sensibly differing from a direct light of the sun, unless when the glasses I used were not sufficiently clear; for then they would a little incline it to their colour.

8. Hence therefore it comes to pass that whiteness is the usual colour of light, for light is a confused aggregate of rays endued with all sorts of colours, as they are promiscuously darted from the various parts of luminous bodies. And of such a confused aggregate, as I said, is generated whiteness, if there be a due proportion of the ingredients; but if any one predominate, the light must incline to that colour, as it happens in the blue flame of brimstone, the yellow flame of a candle, and the various colours of the fixed stars.

9. These things considered, the manner how colours are produced by the prism is evident. For of the rays constituting the incident light, since those which differ in colour proportionally differ in infrangibility, they by their unequal refractions must be severed and dispersed into an oblong form in an orderly succession from the least refracted scarlet to the most refracted violet. And for the same reason it is that objects, when looked upon through a prism, appear coloured. For the difform rays, by their unequal refractions, are made to diverge towards several parts of the retina, and there express the images of things coloured, as in the former case they did the sun's image upon a wall. And by this inequality of refractions they become not only coloured, but also very confused and indistinct.

10. Why the colours of the rainbow appear in falling drops of rain is also from hence evident. For those drops which refract the rays disposed to appear purple in greatest quantity to the spectator's eye, refract the rays of other sorts so much less as to make them pass beside it; and such are the drops on the inside of the primary bow and on the outside of the secondary or exterior one. So those drops which refract in greatest plenty the rays apt to appear red toward the spectator's eye, refract those of other sorts so much more as to make them pass beside it; and such are the drops on the exterior part of the primary and interior part of the secondary bow.

13. I might add more instances of this nature, but I shall conclude with this general one, that the colours of all natural bodies have no other origin than this, that they are variously qualified to reflect one sort of light in greater plenty than another. And this I have experimented in a dark room by illuminating those bodies with uncompounded light of divers colours. For by that means any body may be made to appear of any colour. They have there no appropriate colour, but ever appear of the colour of the light cast upon them, but yet with this difference, that they are most brisk and vivid in the light of their own daylight colour. Minium appeareth there of any colour indifferently with which 'tis illustrated, but yet most luminous in red, and so Bise appeareth indifferently of any colour with which 'tis illustrated, but yet most luminous in blue. And therefore minium reflecteth rays of any colour, but most copiously those endued with red; and consequently when illustrated with daylight, that is, with all sorts of rays promiscuously blended, those qualified with red shall abound most in the reflected light, and by their prevalence cause it to appear of that colour. And for the same reason bise, reflecting blue most copiously, shall appear blue by the excess of those rays in its reflected light; and the like of other bodies. And that this is the entire and adequate cause of their colours is manifest, because they have no power to change or alter the colours of any sort of rays incident apart, but put on all colours indifferently with which they are enlightened.

These things being so it can no longer be disputed whether there be colours in the dark, nor whether they be the qualities of the objects we see, no, nor perhaps whether light be a body. For since colours are the qualities of light, having its rays for their entire and immediate subject, how can we think those rays qualities also, unless one quality may be the subject of and sustain another—which in effect is to call it substance. We should not know bodies for substances were it not for their sensible qualities, and the principal of those being now found due to something else, we have as good reason to believe that to be a substance also.

Besides, who ever thought any quality to be a heterogeneous aggregate, such as light is discovered to be? But to determine more absolutely what light is, after what manner refracted, and by what modes or actions it produceth in our minds the phantasms of colours, is not so easy. And I shall not mingle conjectures with certainties.

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