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| History of theories of Aether and Electricity | |||||||||
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The Theory of Aether in the seventeenth Century
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Cambridge by the intervening Plague " ; this was in 1666, and
his memoir on the subject was not presented to the Royal
Society until five years later. In it he propounds a theory of
colour directly opposed to that of Hooke. " Colours", he says,
"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. " To the same degree of Refrangibility ever belongs the same colour, and to the same colour ever belongs the same degree of Refrangibility." " 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." The publication of the new theory gave rise to an acute controversy. As might have been expected, Hooke was foremost among the opponents, and led the attack with some degree of asperity. When it is remembered that at this time Newton was at the outset of his career, while Hooke was an older man, with an established reputation, such harshness appears particularly ungenerous; and it is likely that the unpleasant consequences which followed the announcement of his first great discovery had much to do with the reluctance which Newton ever afterwards showed to publish his results to the world. In the course of the discussion Newton found occasion to explain more fully the views which he entertained regarding the nature of light. Hooke charged him with holding the doctrine that light is a material substance. Now Newton had, as a matter of fact, a great dislike of the more imaginative kind of hypotheses ; he altogether renounced the attempt to construct the universe from its foundations after the fashion of Descartes, and aspired to nothing more than a formulation of the laws which directly govern the actual phenomena. His theory of gravitation, for example, is strictly an expression of the results of observation, and involves no hypothesis as to the cause of the attraction which subsists between ponderable bodies ; and his own desire in regard to optics was to present a theory free from speculation as to the hidden mechanism of light. Accordingly, in reply to Hooke's criticism, he protested [1] that his views on colour were in no way bound up with any particular conception of the ultimate nature of optical processes. Newton was, however, unable to carry out his plan of connecting together the phenomena of light into a coherent and reasoned whole without having recourse to hypotheses. The hypothesis of Hooke, that light consists in vibrations of an aether, he rejected for reasons which at that time were perfectly cogent, and which indeed were not successfully refuted for over a century. One of these was the incompetence of the wave-theory to account for the rectilinear propagation of light, and another was its inability to embrace the facts discovered, as we shall presently see, by Huygens, and first interpreted correctly by Newton himself of polarization. On the whole, he seems to have favoured a scheme of which the following may be taken as a summary [2]: All space is permeated by an elastic medium or aether, which is capable of propagating vibrations in the same way as the air propagates the vibrations of sound, but with far greater velocity. This aether pervades the pores of all material bodies, and is the cause of their cohesion ; its density varies from one body to another, being greatest in the free interplanetary spaces. It is not necessarily a single uniform substance : but just as air contains aqueous vapour, so the aether may contain various " aethereal spirits," adapted to produce the phenomena of electricity, magnetism, and gravitation. The vibrations of the aether cannot, for the reasons already mentioned, be supposed in themselves to constitute light. Light is therefore taken to be " something of a different kind, propagated from lucid bodies. They, that will, may suppose it an aggregate of various peripatetic qualities. Others may suppose it multitudes of unimaginable small and swift corpuscles of various sizes, springing from shining bodies at great distances one after another; but yet without any sensible interval of time, and continually urged forward by a principle of motion, which in the beginning accelerates them, till the resistance of the aethereal medium equals the force of that principle, much after the manner that bodies let fall in water are accelerated till the resistance of the water equals the force of gravity. But they, that like not this, may suppose light any other corporeal emanation, or any impulse or motion of any other medium or aethereal spirit diffused through the main body of aether, or what else they can imagine proper for this purpose. To avoid dispute, and make this hypothesis general, let every man here take his fancy ; only whatever light be, I suppose it consists of rays differing from one another in contingent circumstances, as bigness, form, or vigour." [3] In any case, light and aether are capable of mutual interaction; aether is in fact the intermediary between light and ponderable matter. When a ray of light meets a stratum of aether denser or rarer than that through which it has lately been passing, it is, in general, deflected from its rectilinear course ; and differences of density of the aether between one material medium and another account on these principles for the reflexion and refraction of light. The condensation or rarefaction of the aether due to a material body extends to some little distance from the surface of the body, so that the inflexion due to it is really continuous, and not abrupt; and this further explains diffraction, which Newton took to be " only a new kind of refraction, caused, perhaps, by the external aethers beginning to grow rarer a little before it came at the opake body, than it was in free spaces." Although the regular vibrations of Newton's aether were not supposed to constitute light, its irregular turbulence seems to have represented fairly closely his conception of heat. He supposed that when light is absorbed by a material body, vibrations are set up in the aether, and are recognizable as the heat which is always generated in such cases. The conduction of heat from hot bodies to contiguous cold ones he conceived to be effected by vibrations of the aether propagated between them ; and he supposed that it is the violent agitation of aethereal motions which excites incandescent substances to emit light. Assuming with Newton that light is not actually constituted by the vibrations of an aether, even though such vibrations may exist in close connexion with it, the most definite and easily conceived supposition is that rays of light are streams of corpuscles emitted by luminous bodies. Although this was not the hypothesis of Descartes himself, it was so thoroughly akin to his general scheme that the scientific men of Newton's generation, who were for the most part deeply imbued with the Cartesian philosophy, instinctively selected it from the wide choice of hypotheses which Newton had offered them ; and by later writers it was generally associated with Newton's name. A curious argument in its favour was drawn from a phenomenon which had then been known for nearly half a century : Vincenzo Cascariolo, a shoemaker of Bologna, had discovered, about 1630, that a substance, which afterwards received the name of Bologna stone or Bologna phosphorus, has the property of shining in the dark after it has been exposed for some time to sunlight ; and the storage of light which seemed to be here involved was more easily explicable on the corpuscular theory than on any other. The evidence in this quarter, however, pointed the other way when it was found that phosphorescent substances do not necessarily emit the same kind of light as that which was used to stimulate them. In accordance with his earliest discovery, Newton considered colour to be an inherent characteristic of light, and inferred that it must be associated with some definite quality of the corpuscles or aether-vibrations. The corpuscles corresponding to different colours would, he remarked, like sonorous bodies of different pitch, excite vibrations of different types in the aether ; and " if by any means those [aether- vibrations] 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." [4] This sentence is the first enunciation of the great principle that homogeneous light is essentially periodic in its nature, and that differences of period correspond to differences of colour. The analogy with Sound is obvious ; and it may be remarked in passing that Newton's theory of periodic vibrations in an elastic medium, which he developed [5] in connexion with the explanation of Sound, would alone entitle him to a place among those who have exercised the greatest influence on the theory of light, even if he had made no direct contribution to the latter subject. Newton devoted considerable attention to the colours of thin, plates, and determined the empirical laws of the phenomena with great accuracy. In order to explain them, he supposed that " every 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." [6] The interval between two consecutive dispositions to easy transmission, or " length of fit", he supposed to depend on the colour, being greatest for red light and least for violet. If then a ray of homogeneous light falls on a thin plate, its fortunes as regards transmission and reflexion at the two surfaces will depend on the relation which the length of fit bears to the thickness of the plate ; and on this basis he built up a theory of the colours of thin plates. It is evident that Newton's "length of fit" corresponds in some measure to the quantity which in the undulatory theory is called the wave-length of the light ; but the suppositions of easy transmission and reflexion were soon found inadequate to explain all Newton's experimental results - at least without making other and more complicated additional assumptions. At the time of the publication of Hooke's Micrographia, and Newton's theory of colours, it was not known whether light is propagated instantaneously or not. An attempt to settle the question experimentally had been made many years previously by Galileo, [7] who had stationed two men with lanterns at a considerable distance from each other ; one of them was directed to observe when the other uncovered his light, and exhibit his own the moment he perceived it. But the interval of time required by the light for its journey was too small to be perceived in this way ; and the discovery was ultimately made by an astronomer. It was observed in 1675 by Olof Roemer [8] (b. 1644, d. 1710) that the eclipses of the first satellites of Jupiter were apparently affected by an unknown disturbing cause ; the time of the occurrence of the phenomenon was retarded when the earth and Jupiter, in the course of their orbital motions, happened to be most remote from each other, and accelerated in the contrary case. Roemer explained this by supposing that light requires a finite time for its propagation from the satellite to the earth ; and by observations of eclipses, he calculated the interval required for its passage from the sun to the earth (the light-equation, as it is called) to be 11 minutes, [9] Shortly after Roemer's discovery, the wave-theory of light was greatly improved and extended by Christiaan Huygens (b. 1629, d. 1695). Huygens, who at the time was living in Paris, communicated his results in 1678 to Cassini, Roemer, De la Hire, and the other physicists of the French Academy, and prepared a manuscript of considerable length on the subject. This he proposed to translate into Latin, and to publish in that language together with a treatise on the Optics of Telescopes ; but the work of translation making little progress, after a delay of twelve years, he decided to print the work on wave-theory in its original form. In 1690 it appeared at Leyden, [10] under the title Traite de la lumiere ou sont expliquees les causes de ce qui luy arrive dans la reflexion et dans la refraction. Et parti- culierement dans Vetrange refraction du cristal d'Islande. Par C.H.D.Z [11] The truth of Hooke's hypothesis, that light is essentially a form of motion, seemed to Huygens to be proved by the effects observed with burning-glasses ; for in the combustion induced at the focus of the glass, the molecules of bodies are dissociated ; which, as he remarked, must be taken as a certain sign of motion, if, in conformity to the Cartesian philosophy, we seek the cause of all natural phenomena in purely mechanical actions. The question then arises as to whether the motion is that of a medium, as is supposed in Hooke's theory, or whether it may be compared rather to that of a flight of arrows, as in the corpuscular theory. Huygens decided that the former alternative is the only tenable one, since beams of light proceeding in directions inclined to each other do not interfere with each other in any way. Moreover, it had previously been shown by Torricelli that light is transmitted as readily through a vacuum as through air ; and from this Huygens inferred that the medium or aether in which the propagation takes place must penetrate all matter, and be present even in all so-called vacua. The process of wave-propagation he discussed by aid of a principle which was now [12] introduced for the first time, and has since been generally known by his name. It may be stated thus : Consider a wave-front, [13] or locus of disturbance, as it exists at a definite instant t : then each surface-element of the wave-front may be regarded as the source of a secondary wave, which in a homogeneous isotropic medium will be propagated outwards from the surface-element in the form of a sphere whose radius at any subsequent instant t is proportional to (t-t0) ; and the wave-front which represents the whole distur- [1] Phil. Trans, vii, 1672, p. 5086. [2] Cf. Newton's memoir in Phil. Trans, vii, 1672 ; his memoir presented to the Royal Society in December, 1675, which is printed in Birch, iii, p. 247; his Opticks, especially Queries 18, 19, 20, 21, 23, 29; the Scholium at the end of the Principia ; and a letter to Boyle, written in February, 1678-9, which is printed in Horsley's Newtoni Opera, p. 385. In the Principia, Book I., section xiv, the analogy between rays of light and streams of corpuscles is indicated ; but Newton does not commit himself to any theory of light based on this. [3] Royal Society, Dec. 9, 1675. [4] Phil. Trans, vii (1672), p. 5088. [5] Newton's Principia, Book ii., Props, xliii.-l. [6] Opticks, Book ii., Prop. 12. [7] Discorsi e dimostrazioni matematiche, p. 43 of the Elzevir edition of 1638. [8] Mem. de l'Acad. x. (1666-1699), p. 575. [9] It was soon recognized that Roemer's value was too large ; and the astronomers of the succeeding half-century reduced it to 7 minutes. Delambre, by an investigation whose details appear to have been completely destroyed, published in 1817 the value 493*2 s , from a discussion of eclipses of Jupiter's satellites during the previous 150 years. Glasenapp, in an inaugural dissertation published in 1875, discussed the eclipses of the first satellite between 1848 and 1870, and derived, by different assumptions, values between 496 s and 501 s , the most probable value being 500*8 s . Sampson, in 1909, derived 498'64 S from his own readings of the Harvard Observations, and 498*79 s from the Harvard readings, with probable errors of about +- 0'02". The inequalities of Jupiter's surface give rise to some difficulty in exact determinations. [10] Huygens had by this time returned to Holland. [11] i.e. Christiaan Huygens de Zuylichem. The custom of indicating names by initials was not unusual in that age. [12] Traite de la lum., p. 17. [13] It maybe remarked that Huygens' " waves " are really what modern writers, following Hooke, call " pulses "; Huygens never considered true wave-trains having the property of periodicity. |