A History of the Theories of Aether and Electricity (1910) — part 7 of 29

" When," he remarks, " M. Oersted discovered the action which a current exercises on a magnet, one might certainly have suspected the existence of a mutual action between two circuits carrying currents ; but this was not a necessary consequence ; for a bar of soft iron also acts on a magnetized needle, although there is no mutual action between two bars of soft iron."

Ampere, therefore, submitted the matter to the test of the laboratory, and discovered that circuits carrying electric currents exert ponderomotive forces on each other, and that

• Recucil d' observations electro-dunamiques, pp. 297, 300, 371.

Galvanism, from Gaivani to Ohm. 89

ponderomotive forces are exerted on such currents by magnets. To the science which deals with the mutual action of currents he gave the name electro-dynamics ;* and he showed that the action obeys the following laws : —

(1) The effect of a current is reversed when the direction of the current is reversed.

(2) The effect of a current flowing in a circuit twisted into small sinuosities is the same as if the circuit were smoothed out.

(3) The force exerted by a closed, circuit on an element of another circuit is at right angles to the latter.

(4) The force between two elements of circuits is unaffected when all linear dimensions are increased proportionately, the current-strengths remaining unaltered.

From these data, together with his assumption that the force between two elements of circuits acts along the line joining them, Ampere obtained an expression of this force : the deduction may be made in the following way : —

Let ds, ds' be the elements, r the line joining them, and i, i' the current-strengths. From (2) we see that the effect of ds on ds' is the vector sum of the effects of dx, dy, dz on ds', where these are the three components of ds: so the required force must be of the form —

r x a scalar quantity which is linear and homogeneous in ds ; and it must similarly be linear and homogeneous in ds' ; so using (1), we see that the force must be of the form

F = ill | (ds . ds') 4> (r) + (ds . r) (ds'. r) i/, (r)} , where <£ and i// denote undetermined functions of r.

From (4) it follows that when ds, ds', r are all multiplied by the same number, F is unaffected : this shows that

4>(r) = - and f (r) = - ,

where A and B denote constants. Thus we have

, M(ds.ds') £(ds.r)(ds'. r))

F = n r \ + - — —- — ; •

( r3 r6 )

*. Loc. cit., p. 298.

90 Galvanism , from Galvani to Ohm.

Now, by (3), the resolved part of F along ds' must vanish when integrated round the circuit s, i.e. it must be a complete differential when dr is taken to be equal to - ds. That is to- say,

^(ds.ds')(r.ds') £(ds . r) (ds'. r)2

/o-»3 .f>£

must be a complete differential ; or

must be a complete differential ; and therefore

7 A BiA

d'^ = --5(dS'r)>

3^ B J

or ~2^" dr = r*dT'

or B = - I A.

Thus finally we have

F = Constant x ii'i || (ds . ds') - -5 (ds . r)(ds'. r)

This is Ampere's formula : the multiplicative constant depends of course on the units chosen, and may be taken to be - 1.

The weakness of Ampere's work evidently lies in the assumption that the force is directed along the line joining the- two elements : for in the analogous case of the action between two magnetic molecules, we know that the force is not directed along the line joining the molecules. It is therefore of interest to find the form of F when this restriction is removed.

For this purpose we observe that we can add to the expression already found for F any term of the form

0(r) . (ds . r) . ds', where 0(r) denotes any arbitrary function of r ; for since

this term vanishes when integrated round the circuit s ; and it

Galvanism, from Galvani to Ohm. 91

contains ds and ds' linearly and homogeneously, as it should. We can also add any terms of the form

rf{r..(ds'.r).x(r)|,

where (r] denotes any arbitrary function of r, and d denotes differentiation along the arc s, keeping ds' fixed (so that dr = - ds) ; this differential may be written

• ds . (ds'. r) . x(r) - rx(r) (ds'. ds) - * x'(r) r (ds . r) (ds'. r).

In order that the law of Action and Eeaction may not be violated, we must combine this with the former additional term so as to obtain an expression symmetrical in ds and ds' : and hence we see finally that the general value of F is given by the equation

F = -n'rjj|(ds.ds')-J(ds.r)(ds.r)j

• x(»-; (ds' . r) ds + x(r) (ds . r) . ds' + x(r) (ds . ds')r

• ix'(r)(ds.r)(ds'.r)r. The simplest form of this expression is obtained by taking

when we obtain

• •/ F = - {(ds . r) . ds' + (ds'. r)ds - (ds . ds')r} .

The comparatively simple expression in brackets is the vector part of the quaternion product of the three vectors ds, r, ds'.*

From any of these values of F we can find the ponderomotive force exerted by the whole circuit s on the element ds' : it is, in fact, from the last expression,

u'f1

[?-3((ds'.r).ds-(ds.ds>},

• The simpler form of F given in the text is, if the term in da' be omitted, the form given by Grassmann, Ann. d. Phys. Ixiv (1845), p. 1. For further work on this subject cf. Tait, Proc. R. S. Edin. viii (1873), p. 220, and Korteweg, Journal fiir Math, xc (1881), p. 45.

92 Galvanism, from Galvani to Ohm.

or i [ds'. B],

where B =

Now this value of B is precisely the value found by Biot and Savart* for the" magnetic intensity at ds' due to the- current i in the circuit s. Thus we see that the ponderomotive force on a current-element ds' in a magnetic field B is i' [ds'. B].

Ampere developed to a considerable extent the theory of the equivalence of magnets with circuits carrying currents; and showed that an electric current is equivalent, in its magnetic effects, to a distribution of magnetism on any surface terminated by the circuit, the axes of the magnetic molecules being everywhere normal to this surface :f such a magnetized surface is called a mayiwtic shell. He preferred, I however, to regard the current rather than the magnetic fluid as the fundamental entity, and considered magnetism to be really an electrical phenomenon : each magnetic molecule owes its properties, according to this view, to the presence within it / of a small closed circuit in which an electric current is perpetually flowing.

The impression produced by Ampere's memoir was great and lasting. Writing half a century afterwards, Maxwell speaks of it as " one of the most brilliant achievements in science." " The whole," he says, " theory and experiment, seems as if it had leaped, full-grown and full-armed, from the brain of the ' Newton of electricity/ It is perfect in form and unassailable in accuracy ; and it is summed up in a formula from which all the phenomena may be deduced, and which must always remain the cardinal formula of electrodynamics."

Not long after the discovery by Oersted of the connexion between galvanism and magnetism, a connexion was discovered between galvanism and heat.| In 1822 Thomas Johann Seebeck

• See ante, p. 86. t Loc. cit., p. 367.

Galvanism, from ^Galvani to Ohm. 93

(b. 1770, d. 1831), of Berlin discovered* that an electric current can be set up in a circuit of metals, without the interposition of any liquid, merely by disturbing the equilibrium of temperature. Let a ring be formed of copper and bismuth soldered together at the two extremities; to establish a current it is only necessary to heat the ring at one of these junctions. To this new class of circuits the name thermo- electric was given.

It was found that the metals can be arranged as a thermo-electric series, in the order of their power of generating currents when thus paired, and that this order is quite different from Volta's order of electromotive potency. Indeed antimony and bismuth, which are near each other in the latter series, are at opposite extremities of the former.

The currents generated by thermo-electric means are generally feeble : and the mention of this fact brings us to the question, which was about this time engaging attention, of the efficacy of different voltaic arrangements.

Comparisons of a rough kind had been instituted soon after the discovery of the pile. The French chemists Antoine FranQois de Fourcroy (b. 1755, d. 1809), Louis Mcolas Yauquelin (b. 1763, d. 1829), and Louis Jacques Thenard (b. 1777, d. 1857) foundf in 1801, on varying the size of the metallic disks constituting the pile, that the sensations produced on the human frame were unaffected so long as the number of disks remained the same; but that the power 'of burning finely drawn wire was altered; and that the latter power was proportional to the total surface of the disks employed, whether this were distributed among a small number of large disks, or a large number of small ones. This was

• Abhandl. d. Berlin Akad. 1822-3 ; Ann. d. Phys. Ixxiii (1823), pp. 115, 430 ; vi (1826), pp. 1, 133, 253.

Volta had previously noticed that a silver plate whose ends were at different temperatures appeared to act like a voltaic cell.

Further experiments were performed by James Gumming (£. 1777, d. 1861), Professor of Chemistry at Cambridge, Trans. Camb. Phil. Soc. ii (1823), p. 47, and by Antoine Cesar Becquerel (b. 1788, d. 1878), Annales de Chimie, xxxi (1826), p. 371. t Ann. de Chimie, xxxix (1801), p. 103.

94 Galvanism, from Galvani to Ohm.

explained by supposing that small plates give a small quantity of the electric fluid with a high velocity, while large plates give a larger quantity with no greater velocity. Shocks, which were supposed to depend on the velocity of the fluid alone, would therefore not be intensified by increasing the size of the plates.

The effect of varying the conductors which connect the terminals of the pile was also studied. Nicolas Grautherot (b. 1753, d. 1803) observed* that water contained in tubes which have a narrow opening does not conduct voltaic currents so well as when the opening is more considerable. This experi- ment is evidently very similar to that which Beccaria had performed half a century previously! with electrostatic discharges.

As we have already seen, Cavendish investigated very -completely the power of metals to conduct electrostatic discharges; their power of conducting voltaic currents was now examined by Davy.J His method was to connect the terminals of a voltaic battery by a path containing water (which it decomposed), and also by an alternative path consisting of the metallic wire under examination. When the length of the wire was less than a certain quantity, the water ceased to be decomposed ; Davy measured the lengths and weights of wires of different materials and cross-sections under these limiting circumstances ; and, by comparing them, showed that the conducting power of a wire formed of any one metal is inversely proportional to its length and directly proportional to its sectional area, but independent of the shape of the cross- section.! The latter fact, as he remarked, showed that voltaic currents pass through the substance of the conductor and not along its surface.

Davy, in the same memoir, compared the conductivities of various metals, and studied the effect of temperature : he found

• Annales de China., xxxix (1801), p. 203. t See p. 53.

% Phil. Trans., 1821, p. 433. His results were confirmed afterwards by Becquerel, Annales de Chiiuie, xxxii (1825), p. 423. 6 These results had been known to Cavendish.

Galvanism , from Galvani to Ohm. 95

that the conductivity varied with the temperature, being " lower in some inverse ratio as the temperature was higher."

He also observed that the same magnetic power is exhibited by every part of the same circuit, even though it be formed of wires of different conducting powers pieced into a chain, so that " the magnetism seems directly as the quantity of electricity which they transmit."

The current which flows in a given voltaic circuit evidently depends not only on the conductors which form the circuit, but also on the driving-power of the battery. In order to form a complete theory of voltaic circuits, it was therefore necessary to extend Davy's laws by taking the driving-power into account. This advance was effected in 1826 by Georg Simom Ohm* (b. 1787, d. 1854).

Ohm had already carried out a considerable amount of experimental work on the subject, and had, e.g., discovered that if a number of voltaic cells are placed in series in a circuit, the current is proportional to their number if the external resistance is very large, but is independent of their number if the external resistance is small. He now essayed the task of combining all the known results into a consistent theory.

For this purpose he adopted the idea of comparing the flow of electricity in a current to the flow of heat along a wire, the theory of which had been familiar to all physicists since the publication of Fourier's Theorie analytique de la chcdeur in 1822. " I have proceeded," he says, " from the supposition that the communication of the electricity from one particle takes place directly only to the one next to it, so that no immediate transition from that particle to any other situate at a greater distance occurs. The magnitude of the flow between two adjacent particles, under otherwise exactly similar circum- stances, I have assumed to be proportional to the difference of

*Ann. d. Phys. vi (1826), p. 459 ; vii, pp. 45,117; Die Galvanische Eette mathematisch bearbeitet : Berlin, 1827 ; translated in Taylor's Scientific Memoirs, ii (1841), p. 401. Cf. also subsequent papers by Ohm in Kastner's Archiv fur d. ges. Naturkhre, and Schweigger's Jahrbuch.

96 • Galvanism, from Galvani to Ohm.

the electric forces existing in the two particles ; just as, in the theory of heat, the flow of caloric between two particles is regarded as proportional to the difference of their temperatures."' '*' The comparison between the flow of electricity and the flow of heat suggested the propriety of introducing a quantity whose behaviour in electrical problems should resemble that of temperature in the theory of heat. The differences in the values of such a quantity at two points of a circuit would provide what was so much needed, namely, a measure of the "driving-power" acting on the electricity between these points. To carry out this idea, Ohm recurred to Volta's theory of the electrostatic condition of the open pile. It was cus- tomary to measure the " tension " of a pile by connecting one terminal to earth and testing the other terminal by an electroscope. Accordingly Ohm says : " In order to investigate the changes which occur in the electric condition of a body A in a perfectly definite manner, the body is each time brought, under similar circumstances, into relation with a second moveable body of invariable electrical condition, called the electroscope ; and the force with which the electroscope is repelled or attracted by the body is determined. This force is termed the electroscopic force of the body A"

" The same body A may also serve to determine the electro- scopic force in various parts of the same body. For this purpose take the body A of very small dimensions, so that when we bring it into contact with the part to be tested of any third body, it may from its smallness be regarded as a substitute for this part : then its electroscopic force, measured in the way described, will, when it happens to be different at the various places, make known the relative differences with regard to electricity between these places."

Ohm assumed, as was customary at that period, that when two metals are placed in contact, " they constantly maintain at the point of contact the same difference between their electro- scopic forces." He accordingly supposed that each voltaic cell possesses a definite tension, or discontinuity of electroscopic

Galvanism, from Galvani to Ohm. 97

force, which is to be regarded as its contribution to the driving- force of any circuit in which it may be placed. This assumption confers a definite meaning on his use of the term " electroscopic force " ; the force in question is identical with the electrostatic potential. But Ohm and his contemporaries did not correctly understand the relation of galvanic conceptions to the j electrostatic functions of Poisson. The electroscopic force in the open pile was generally identified with the thickness of the electrical stratum at the place tested ; while Ohm, recognizing that electric currents are not confined to the surface of the conductors, but penetrate their substance, seems to have thought of the electroscopic force at a place in a circuit as being proportional to the volume-density of electricity there — an idea in which he was confirmed by the relation which, in an analogous case, exists between the temperature of a body and the volume-density of heat supposed to be contained in it.

Denoting, then, by S the current which flows in a wire of conductivity y, when the difference of the electroscopic forces at the terminals is E, Ohm writes

S = yE.

From this formula it is easy to deduce the laws already given by Davy. Thus, if the area of the cross-section of a wire is Ay we can by placing n such wires side by side construct a wire of cross-section nA. If the quantity E is the same for each, equal currents will flow in the wires ; and therefore the current in the compound wire will be ?i times that in the single wire ; so when the quantity E is unchanged, the current is proportional to the cross-section; that is, the conductivity of a wire is directly proportional to its cross-section, which is one of Davy's laws.

In spite of the confusion which was attached to the idea of electroscopic force, and which was not dispelled for some years, the publication of Ohm's memoir marked a great advance in electrical philosophy. It was now clearly understood that the current flowing in any conductor depends only on the

H

98 Galvanism^ from Galvani to Ohm.

conductivity inherent in the conductor and on another variable which bears to electricity the same relation that temperature bears to heat ; and, moreover, it was realized that this latter variable is the link connecting the theory of currents with the older theory of electrostatics. These principles were a sufficient foundation for future progress; and much of the work which was published in the second quarter of the century was no more than the natural development of- the principles laid down by Ohm.*

It is painful to relate that the discoverer had long to wait before the merits of his great achievement were officially recognized. Twenty- two years after the publication of the memoir on the galvanic circuit, he was promoted to a university professorship ; this he held for the five years which remained until his death in 1854.

• Ohm's theory was confirmed experimentally by several investigators, among whom may be mentioned Gustav Theodor Feehner(i. 1801, d. 1887) (Maassbestim- mungen iiber die Galvanische Kette, Leipzig, 1831), and Charles Wheatstone (b. 1802, d. 1875) (Phil. Trans,, 1843, p. 303).

CHAPTER IV.

THE LUMINIFEROUS MEDIUM, FROM BRADLEY TO FRESNEL.

ALTHOUGH Newton, as we have seen, refrained from committing himself to any doctrine regarding the ultimate nature of light, the writers of the next generation interpreted his criticism of the wave-theory as equivalent to an acceptance of the corpuscular hypothesis. As it happened, the chief optical discovery of this period tended to support the latter theory, by which it was first and most readily explained. In 1728 James Bradley (b. 1692, d. 1762), at that time Savilian Professor of Astronomy at Oxford, sent to the Astronomer Royal (Halley) an " Account of a new discovered motion of the Fix'd Stars."* In observing the star y in the head of the Dragon, he had found that during the winter of 1725-6 the transit across the meridian was continually more southerly, while during the following summer its original position was restored by a motion northwards. Such an effect could not be explained as a result of parallax ; and eventually Bradley guessed it to be due to the gradual propagation of light.f

Thus, let CA denote a ray of light, falling on the line BA ; and suppose that the eye of the observer is travelling ^ along BA, with a velocity which is to the velocity of light as BA is to CA. Then the corpuscle of light, by which the object is discernible to the eye at A, would have been at C when the eye was at B. The tube of a telescope must therefore be pointed in the direction BC, in order to receive the rays from an object whose light is really propagated in the direction CA. The angle BCA measures the difference between the real and apparent positions »

of the object ; and it is evident from the figure that the sine of

•Phil. Trans, xxxv (1728), p. 637.

t Roemer, in a letter to Huygens of date 30th Dec., 1677, mentions a suspected displacement of the apparent position of a star, due to the motion of the earth at right angles to the line of sight. Cf . Correspondance de Huygens, viii, p. 53.

H 2

100 The Lumini/erous Medium,

this angle is to the sine of the visible inclination of the object to the line in which the eye is moving, as the velocity of the eye is to the velocity of light. Observations such as Bradley's will therefore enable us to deduce the ratio of the mean orbital velocity of the earth to the velocity of light, or, as it is called,. the constant of 'aberration ; from its value Bradley calculated that light is propagated from the sun to the earth in 8 minutes 12 seconds, which, as he remarked, "is as it were a Mean betwixt what had at different times been determined from the eclipses of Jupiter's satellites."*

With the exception of Bradley's discovery, which was primarily astronomical rather than optical, the eighteenth century was decidedly barren, as regards both the experimental and the theoretical investigation of light ; in curious contrast to the brilliance of its record in respect of electrical researches. But some attention must be given to a suggestive study f of the aether, for which the younger John Bernoulli (b. 1710, d. 1790) was in 1736 awarded the prize of the French Academy. His ideas seem to have been originally suggested by an attempt};

*Struve in 1845 found for the constant of aberration the value 20"'445, which lie afterwards corrected to 20"'463. This was superseded in 1883 by the value 20"-492, determined by M. Nyren. The observations of both Struve and Nyren were made with the transit in the prime vertical. The method now generally used depends on the measurement of differences of meridian zenith distances (Talcott's method, as applied by F. Kiistner, Beobachtungs-Ergebnisse der kon. Stern warte zu Berlin, Heft 3, 1888) ; the value at present favoured for the constant of aberration is 20"-523. Cf. Chandler, Ast. Journal, xxiii, pp. 1, 12 (1903).

The collective translatory motion of the solar system gives rise to aberrational: terms in the apparent places of the fixed stars ; but the principal term of this character does not vary with the time, and consequently is equivalent to a permanent constant displacement. The second-order terms (i.e. those which involve the ordinary constant of aberration multiplied by the sun's velocity) might be measurable quantities in the case of stars near the Pole ; and the same is true of the variations in the first-order terms (i.e. those which involve the sun's velocity not multiplied by the constant of aberration) due to the circumstance that the star's apparent R. A. and Declination, which occur in these terms, are not constant, but are affected by Precession, Nutation, and Aberration. Cf. Seeliger, Ast. Nach., cix., p. 273 (1884).

t Printed in 1752, in the Recueil des pieces qui ont remportes les prix de V Acad.y. tome iii. J Acta eniditorum, MDCCI, p. 19.

from Bradky to Fresnel. 101

which his father, the elder John Bernoulli (b. 1667, d. 1748), had made in 1701 to connect the law of refraction with the mechanical principle of the composition of forces. If two opposed forces whose ratio is ju maintain in equilibrium a particle which is free to move only in a given plane, it follows from the triangle of forces that the directions of the forces must obey the relation

sin i = fj. sin r,

where i and r denote the angles made by these directions with the normals to the plane. This is the same equation as that which expresses the law of refraction, and the elder Bernoulli conjectured that a theory of light might be based on it ; but he gave no satisfactory physical reason for the existence of forces along the incident and refracted rays. This defect his son now proceeded to remove.

All space, according to the younger Bernoulli, is permeated by a fluid aether, containing an immense number of excessively small whirlpools. The elasticity which the aether appears to possess, and in virtue of which it is able to transmit vibrations, is really due to the presence of these whirlpools ; for, owing to -centrifugal force, each whirlpool is continually striving to dilate, and so presses against the neighbouring whirlpools. It will be seen that Bernoulli is a thorough Cartesian in spirit ; not only does he reject action at a distance, but he insists that •even the elasticity of his aether shall be explicable in terms of matter and motion.

This aggregate of small vortices, or " fine-grained turbulent motion," as it came to be called a century and a half later,* is interspersed with solid corpuscles, whose dimensions are small -compared with their distances apart. These are pushed about by the whirlpools whenever the aether is disturbed, but never travel far from their original positions.

A source of light communicates to its surroundings a disturbance which condenses the nearest whirlpools ; these by

• Cf . Lord Kelvin's vortex-sponge aether, described later in this work.

102 The Luminiferous Medium i

their condensation displace the contiguous corpuscles from their equilibrium position ; and these in turn produce condensations in the whirlpools next beyond them, so that vibrations are propagated in every direction from the luminous point. It is curious that Bernoulli speaks of these vibrations as longitudinal, and actually contrasts them with those of a stretched cord, which, " when it is slightly displaced from its rectilinear form, and then let go, performs transverse vibrations in a direction at right angles to the direction of the cord." When it is remembered that the objection to longitudinal vibrations, on the score of polarization, had already been clearly stated by Newton, and that Bernoulli's aether closely resembles that which Maxwell invented in 1861-2 for the express purpose of securing transversality of vibration, one feels that perhaps no man ever so narrowly missed a great discovery.

Bernoulli explained refraction by combining these ideas with those of his father. Within the pores of ponderable bodies the whirlpools are compressed, so the centrifugal force must vary in intensity from one medium to another. Thus a corpuscle situated in the interface between two media is acted on by a greater elastic force from one medium than from the other; and by applying the triangle of forces to find the- conditions of its equilibrium, the law of Snell and Descartes

r may be obtained.

Not long after this, the echoes of the old controversy

' between Descartes and Fermat about the law of refraction were awakened* by Pierre Louis Moreau deMaupertuis (b. 1698,, d. 1759).

It will be remembered that according to Descartes the velocity of light is greatest in dense media, while according to- Fermat the propagation is swiftest in free aether. The argu- ments of the corpuscular theory convinced Maupertuis that on this particular point Descartes was in the right ; but never- theless he wished to retain for science the beautiful method by which Fermat had derived his result. This he now proposed

*Mem. de 1'Acad., 1744, p. 417.

from Bradley to Fresnel. 103

to do by modifying Fermat's principle so as to make it agree with the corpuscular theory; instead of assuming that light follows the quickest path, he supposed that " the path described is that by which the quantity of action is the least " ; and this action he defined to be proportional to the sum of the spaces described, each multiplied by the velocity with which it is traversed. Thus instead of Fermat's expression

dt or

tds

} v

(where t denotes time, v velocity, and ds an element of the path) Maupertuis introduced

/v ds

as the quantity which is to assume its minimum value when the path of integration is the actual path of the light. Since Maupertuis' v, which denotes the velocity according to the corpuscular theory, is proportional to the reciprocal of Fermat's v, which denotes the velocity according to the wave- theory, the two expressions are really equivalent, and lead to the same law of refraction. Maupertuis' memoir is, however, of great interest from the point of view of dynamics ; for his suggestion was subsequently developed by himself and by Euler and Lagrange into a general principle which covers the whole range of Nature, so far as Nature is a dynamical system.

The natural philosophers of the eighteenth century for the most part, like Maupertuis, accepted the corpuscular hypothesis ; 'but the wave-theory was not without defenders. Franklin* declared for it ; and the celebrated mathematician Leonhard Euler (b. 1707, d. 1783) ranged himself on the same side. In a work entitled Nova Theoria Lucis et Colorum, published! while he was living under the patronage of Frederic the Great at Berlin, he insisted strongly on the resemblance between light and sound ; " light is in the aether the same thing as sound in air/' Accepting Newton's doctrine that colour depends on

• Letter xxiii, written in 1752.

tL. Euleri Opuscula varii argumenti, Berlin, 1746, p. 169.

104 The Luminiferous Medium ,

wave-length, he in this memoir supposed the frequency greatest for red light, and least for violet ; but a few years later* he adopted the opposite opinion.

The chief novelty of Euler's writings on light is his explanation of the manner in which material bodies appear coloured when viewed by white light ; and, in particular, of the way in which the colours of thin plates are produced. He denied that such colours are due to a more copious reflexion of light of certain particular periods, and supposed that they represent vibrations generated within the body itself under the stimulus of the incident light. A coloured surface, according to this hypothesis, contains large numbers of elastic molecules, which, when agitated, emit light of period depending only on their own structure. The colours of thin plates Euler explained in the same way ; the elastic response and free period of the plate at any place would, he conceived, depend on its thickness at that place ; and in this way the dependence of the colour on the thickness was accounted for, the phenomena as a whole being analogous to well-known effects observed in experiments on sound.

An attempt to improve the corpuscular theory in another direction was made in 1752 by the Marquis de Courtivron,f and independently in the following year by T. Melville These writers suggested, as an explanation of the different refran- gibility of different colours, that " the differently colour'd rays are projected with different velocities from the luminous body : the red with the greatest, violet with the least, and the inter- mediate colours with intermediate degrees of velocity." On this supposition, as its authors pointed out, the amount of aberration would be different for every different colour ; and the satellites of Jupiter would change colour, from white through green to violet, through an interval of more than half a minute before their immersion into the planet's shadow ; while at emersion the contrary succession of colours should be observed,

• Mem. del' Acad.de Berlin, 1752, p. 262. t Courtivron's Traite cfoptique, 1752. JPhil. Trans, xlviii (1753), p. 262.

from Bradley to FremeL 105

beginning with red and ending in white. The testimony of practical astronomers was soon given that such appearances are not observed ; and the hypothesis was accordingly abandoned.

The fortunes of the wave-theory began to brighten at the end of the century, when a new champion arose. Thomas Young, born at Milverton in Somersetshire in 1773, and trained to the practice of medicine, began to write on optical theory in 1799. In his first paper* he remarked that, according'1 to the corpuscular theory, the velocity of emission of a corpuscle must be the same in all cases, whether the projecting force be that of the feeble spark produced by the friction of two Q pebbles, or the intense heat of the sun itself — a thing almost incredible. This difficulty does not exist in the undulatory theory, since all disturbances are known to be transmitted^ through an elastic fluid with the same velocity. The reluctance which some philosophers felt to filling all space with an elastic fluid he met with an argument which strangely foreshadows the electric theory of light : " That a medium resembling in many properties that which has been denominated ether does really exist, is undeniably proved by the phenomena of electricity. The rapid transmission of the electrical shock shows that the electric medium is possessed of an elasticity as great as is necessary to be supposed for the propagation of light. Whether the electric ether is to be considered the same with the luminous ether, if such a fluid exists, may perhaps at some future time be discovered by experiment : hitherto I have not been able to observe that the refractive power of a fluid undergoes any change by electricity."