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is no doubt the consequence of the exclusion of diagrams; for it is impossible, by any description, however vivid or circuitous, to convey to the mind a just idea of the exquisite symmetry which characterises the movements of vibrating solids, and which depends on the division of the body by nodal and neutral lines marking the points and lines of repose, and separating the segments which are in a state of depression, from those which are in a state of elevation. There is no branch of physics which addresses itself so agreeably to the eye, or appeals with such force to our wonder, as that of acoustic figures; and, connected as it is with the theory and practice of music, we must implore Mrs Somerville to give it, in another edition, a more favourable consideration. The delineations of Chladni and Savart are highly interesting; and the beautiful analysis of the whole class of phenomena which Mr Wheatstone has given in the last Part of the Philoso'phical Transactions,' and illustrated with about 200 or 300 figures, gives a fresh importance to this branch of physics.
Mrs Somerville next proceeds to the science of Physical Optics, in which she treats, in the compass of eighty pages, of the subjects of inequal refraction, the composition and decomposition of light, accidental colours, the interference of rays, the colours of thin plates, the undulatory theory, and the polarisation and double refraction of light; all of which she discusses with her usual ability, and with the same correct knowledge of the subject which she displays in her own more peculiar department of Astronomy. Among the more recent enquiries on the subject of light, Mrs Somerville gives a brief account of those of M. Plateau of Brussels, the ingenious improver of that beautiful instrument called the Phenakistiscope. The subject to which we refer is that of accidental colours, respecting which the Flemish philosopher is supposed to have made some important discoveries.
Recent experiments by Plateau of Brussels prove that direct and accidental colours differ essentially. From these it appears that two complementary colours from direct impression, which would produce white when combined, produce black, or extinguish one another by their union, when accidental; and also that the combination of all the tints of the solar spectrum produces white light if they be from a direct impression on the eye, whereas blackness results from a union of the same tints if they be accidental. M. Plateau attributes the phenomena of accidental colours to a reaction of the retina after being excited by direct vision. When the image of an object is impressed on the retina only for a few moments, the picture left is exactly of the same colour with the object, but in an extremely short time the picture is succeeded by the accidental image. If the prevailing impression be a very strong white light, its accidental image is not black, but a variety of colours in
succession. With a little attention it will generally be found that, whenever the eye is affected by one prevailing colour, it sees at the same time the accidental colour, in the same manner as in music the ear is sensible at once to the fundamental note and its harmonic sounds. The imagination has a powerful influence on our optical impressions, and has been known to revive the images of highly luminous objects months and even years afterwards.'
The general reader of this passage, as well as of M. Plateau's original memoir, can scarcely fail to be struck with the extraordinary announcement that blackness arises from the union of all the colours of the spectrum, when they are the accidental colours,* and the discovery must appear to him an important one. A slight examination of the subject, however, will show us that this position is a verbal illusion; and that the physical fact which it so erroneously expresses, has been long known to philosophers.
An accidental colour is something essentially distinct from a colour produced by the action of direct rays. The rays which produce ordinary colours, can be combined in any proportion we please; and the resulting effect is the sum of the actions of each separate ray upon the retina. Hence all the different colours of the spectrum produce a purely white beam of light; and perfect whiteness may also be produced by two compound colours, one of which is complementary to the other. An accidental colour, however, cannot be added to, or combined with, another. When the eye sees an accidental colour, suppose red, the excited part of the retina is insensible to all other rays but those of the accidental colour. If we instantly excite the same portion of the retina with another light which is an accidental green, and thus render it insensible to red, then the eye will see blackness, not because the accidental red and the accidental green compose blackness, but because the eye has been in succession rendered insensible to the two colours which compose white light itself. If Buffon, or Dr Darwin, or Count Rumford, had been asked what would be the effect of exciting the retina in quick succession with all the simple colours of the spectrum, or with two compound colours which compose white light, they would all have immediately answered, blackness. M. Plateau has therefore, in this part of the
* Plateau's own words are
Dans le cas où la combinaison des couleurs réelles produit le blanc, la combinaison des couleurs accidentelles produit l'opposé du blanc, ou le noir. Par example, tandis que deux couleurs réelles complémentaires produisent ensemble du blanc, deux couleurs accidentelles complémentaires produisent ensemble du noir.-Ann. Chim. Aug. 1833, p. 388.
enquiry, merely expressed what has been long known, in language physically incorrect, and calculated to convey very erroneous notions of the subject.
In the rest of the passage which we have quoted, Mrs Somerville has given an imperfect account of Plateau's theory of accidental colours, combining it with another theory long ago published;* namely, that the accidental colour is seen at the same time with the primitive colour, in the same manner as the ear hears at once the fundamental note and its harmonic sounds. Plateau, maintains, that after the direct or positive impression of the primitive colour has continued visible for a certain time, and gradually faded away, it is succeeded by the negative impression, or accidental colour. But, what is original in his theory, he maintains, that after the accidental colour has faded away in its turn, it is succeeded by the primitive colour, this alternation going on till the impression wears away. If we look into the volume already quoted,t we shall see that the only novelty in Plateau's theory is the recurrence of the primitive impression, and the continued alternation of the two; but we do not think that this recurrence and alternation are established by sufficient evidence,―at least, we cannot by any contrivance render it visible. It is certain that the accidental colour disappears and returns, and undergoes other changes; but these changes, we conceive, are not the effect of the primitive impression, or a continuation of a necessary series of changes, of which the primitive impression is the commencement; but the result of subsequent actions upon the retina, which M. Plateau has not been careful enough to detect and analyze. It has been proved, for example, that a pressure upon the retina with the finger changes the accidental colour;+ and it is asserted by Sir Charles Bell, that if we squint or distort the eye, a vivid impression on the retina instantly disappears, as if it were wiped out. When M. Plateau, therefore, saw the accidental colour change into the primitive, was he sure that there was no pressure made upon the retina by the motion of the eye, or by the involuntary closing even of the eyelids, which is sufficient of itself to produce the observed change? That the changes of colour in question are not regular, and are produced by some irregular influence, may be inferred from M. Plateau's own observation, that the alternations of colour do not always take place in the same manner; that they vary with the sensibility of the eyes, and particularly with the circumstances under
* Treatise on Optics, Cab. Cyclop. p. 299, 309.. + Ibid. p. 300. Lond. and Edin. Phil. Mag., Aug. 1832, No. II. p. 91.
"which the experiment is made;' and he afterwards remarks, that the regular alternation of the primitive and accidental colour is 'the effect most frequently observed.' Now, this additional frequency of one phenomenon in a series is no proof of a regular law; and when we consider how the retina is affected by the state of the stomach, by the pressure of the blood-vessels, which may, in some cases, be an intermitting or an alternating one, we must demand a series of distinct experiments made with the same result, on the eyes of different observers accustomed to the examination of this class of phenomena, and aware of the causes which exercise a disturbing influence, before we can admit the conclusion drawn by M. Plateau. There can be no doubt that the accidental colour vanishes and reappears; and Sir David Brewster has observed the curious fact, not only that the acci dental colour in its brightest phase may be made to disappear by a smart blow upon the head, above the excited eye, but that it may be prevented from at first appearing, by giving the blow to the head immediately before the eye is withdrawn from the primitive colour.
In the next section of her work, Mrs Somerville proceeds to the subject of the interference of light, and gives a very brief but distinct view of the undulatory theory. Mrs Somerville is a decided undulationist; and her belief in the existence of a resisting ether in the planetary spaces has increased since the publication of her larger work; or rather, perhaps, it is more strongly expressed in the present volume. We are ourselves great admirers of the undulatory doctrine, and have always regarded it as a theory advancing rapidly to completion; but we view it only as a theory, and dislike extremely the dogmatism with which it is sometimes supported. After describing the beautiful experiment of Dr Young, in which the dark fringes within the shadows of bodies are shown to depend on the interference of rays passing on each side of the body, Mrs Somerville observes, that it is contrary 'to all our ideas of matter to suppose that two particles of it 'should annihilate one another under any circumstances what'ever;' and she afterwards adds, that the preceding experi'ments, and the inferences deduced from them, which have led to ❝the establishment of the doctrine of the undulation of light, are 'the most splendid memorials of our illustrious countryman, 'Thomas Young, though Huygens was the first to originate the 'idea.'
Now, it is obvious that Mrs Somerville, and others who have recently written on the subject, are not at all aware that Dr Young, though hostile to the Newtonian theory of emission, has, with the utmost candour, declared that the law of interference
may be reconciled to the doctrine of emission; if we suppose, with Newton, that the projected corpuscles of light excite sensation by means of the vibrations of the fibres of the retina and of the nerves. Hence we may imagine, he says, that such vibrations must be most easily produced by a series of particles following each other at 'equal distances, each colour having its appropriate distance in any given medium; and it will be demonstrable, that a second series of similar particles, interfering with them in such a manner as to bisect the intervals, will destroy their influence in producing a vibratory motion, each succeeding particle meeting the fibre at the instant of its return from the excursion generated by the stroke of the preceding particle. Hence Dr Young conceives, that the motive effect of the stroke will be annihilated.
Now, though it is manifest that two particles of matter cannot, under any circumstances whatever, annihilate each other, yet it is equally certain, and it is sufficient for the purposes of the emissionist, that the motive effect of the stroke of such particles upon the fibres of the retina be annihilated. Mrs Somerville, therefore, is not justified in saying, that the Newtonian theory seems totally inadequate to account for the phenomenon of interference.
In our capacity of reviewers, we are not in the slightest degree concerned with the scientific creed of individual philosophers. Every theorist has a right to the most unlimited toleration; but it is our business to request, and we do it with all humility, that those who write upon the subject of the all-pervading ether,' will not in one page speak of it as a probable hypothesis, and in another, as a demonstrated existence.
Mrs Somerville proceeds, in the following section, to treat of the double refraction and polarization of light. Having experienced how difficult it is to convey, even to the most acute and intelligent persons, a general view of this curious subject, even with the aid of diagrams, and models, and direct experiments, we can scarcely suppose that Mrs Somerville's brief, though perspicuous exposition of it, will be satisfactory to her readers. We shall enable our readers, however, to judge for themselves. The following is Mrs Somerville's account of the polarization of light :
In general, when a ray of light is reflected from a pane of plate-glass, or any other substance, it may be reflected a second time from another surface, and it will also pass freely through transparent bodies; but if a ray of light be reflected from a pane of plate-glass at an angle of 57°, it is rendered totally incapable of reflection at the surface of another pane of glass in certain definite positions, but will be completely reflected by