March 24, 1892] 



NATURE 



Q£ 



499 



not improbable that these linear brightenings in the broad dark 

 lines indicate eruptions of gases from the interior of the body 

 possessing the continuous spectrum with the dark absorption 

 lines. Such brightenings are occasionally seen in the spectra 

 of sun-spots. On this supposition, the fine bright lines would 

 mdicate very nearly the middle of the dark lines. 



" The appear.ince of two maxima of intensity in the broad 

 bright lines admits of the conclusion that two bodies with 

 different motions possess spectra with bright (lines, and that 

 therefore the spectrum of the Nova consists of at least three 

 spectra superposed, from the measurement of which, in connec- 

 tion with the comparison spectra of /3 Aurigse or /3 Tauri on the 

 same plate, the relative motions of the three supposed bodies, 

 a>< well as their motions with respect to the earth, can be deter- 

 mined. Denoting the body with the dark-line spectrum by a, 

 the two others with bright-line spectra by l> and c, measure- 

 ments by myself and Dr. Scheiner have given the following 

 results : 



a - ^{b + c) = 120 miles,! 

 [> - c =■ "JO miles, 

 and further with respect to the earth — 



a = - 90 miles, // = - ^, c — +65 miles. 



" This result is still very uncertain, and must be regarded as 

 quite preliminary, for it is evident that with the small size of the 

 spectra the accuracy cannot be pushed very far — a displacement 

 of "Ol mm. corresponds, for instance, to a motion of 8 to 12 

 miles, according to the situation of the line in the spectrum — 

 and that the size of the silver grain in the photographs can 

 exert a very marked influence on the measurements. 



" In the photographic spectrum of the Nova, besides the 

 broad lines mentioned, several more bright and mostly very 

 broad lines can be seen, whose wave-lengths T intend to com- 

 municate later on. ' 



Prof. Pickering communicates some valuable information to 

 the same number of the Astronomische Nachrichtett with refer- 

 ence to the visibility of the Nova before its discovery by Dr. 

 Anderson. In eighteen photographs of this region, which 

 were taken by the 8-inch photographic telescopes between 

 the dates November 3, 1885, and November 2, 1891, no 

 star in the Nova's place was visible, but in those taken from 

 December 16, 1891, to January 31, 1891, there was a star of the 

 fifth magnitude recorded. In another series of plates taken 

 with the transit photometer, no record of the new star up to 

 December i, 1891, was obtained, although x Aurigae (mag. 

 5'o m.) was always visible, but the plates taken on the nighls of 

 December 10, 1891, and ending January 20, 1892, indicated 

 clearly the position of the new star. 



Careful examination has been made on all the above-mentioned 

 plates, and the following extract shows the series of magnitudes 

 which have been deduced from the measurements : — 



"It appears that the star was fainter than the eleventh mag- 

 nitude on November 2, 1891, than the sixth magnitude on 

 December i, and that it was increasing rapidly on December 10. 

 A graphical construction indicates that it had probably attained 

 the seventh magnitude within a day or two of December 2, and 

 the sixth magnitude December 7. The brightness increased 

 rapidly until December 18, attaining its maximum about Decem- 

 ber 20, when its magnitude was 4'4m. It then began to 

 decrease slowly, with slight fluctuations, until January 20, when 

 it was slightly below the fifth magnitude." 



From this it will be seen that two months' observations have 

 been lost, owing to its late discovery. 



ABERRA TION? 

 T TNDER this head may conveniently be considered not only the 

 ^ apparent displacement of the stars discovered by Bradley, 

 but other kindred phenomena dependent upon the velocity of 

 light bearing but a finite ratio to that of the earth in its orbit 

 round the sun, and to other astronomical velocities. 



The explanation of stellar aberration, as usually given, 



■ = about 540 English miles. — Tr. 



* Thispaper was written in 1887, when I Was occupied with my article upon 

 Wave Theory" for the " Encyclop<edia Britannica," and at a time when 

 a more extensive treatment was contemplated than was afterwards found 

 practicable. Friends on whom I can rely are of opinion that its publication 

 may be useful ; and, as I am not able to give it a complete revision. 1 prefer 

 to let it stand under its original date, merely warning the reader that very 

 important work has since been published by Michelson.— 7>i«?<ao' 1*592- 



NO. II 69, VOL. 45] 



proceeds rather upon the basis of the corpuscular than of the 

 wave theory. In order to adapt it to the principles of the latter 

 theory, Fresnel found it necessary to follow Young in assuming 

 that the sether in any vacuous space connected with the earth 

 (and therefore practically in the atmosphere) is undisturbed by 

 the earth's motion of 19 miles per second. Consider for simpli- 

 city the case in which the direction of the star is at right angles 

 to that of the earth's motion, and replace the telescope, which 

 would be used in practice, by a pair of perforated screens, on 

 ! which the light falls perpendicularly. We may further imagine 

 I the luminous disturbance to consist of a single plane pulse. 

 When this reaches the anterior screen, so much of it as coincides 

 with the momentary position of the aperture is transmitted, and 

 i the remainder is stopped. The part transmitted proceeds upon 

 its course through the ather independently of the motion of the 

 ' screens. In order, therefore, that the pulse may be transmitted 

 by the aperture in the posterior screen, it is evident that the 

 line joining the centres of the apertures must not be perpen- 

 dicular to the screens and to the wave front, as would have been 

 necessary in the case of rest. For in consequence of the 

 motion of the posterior screen in its own plane the aperture 

 will be carried forward during the time of passage of the light. 

 By the amount of this motion the second aperture must be 

 drawn backwards, in order that it may be in the place required 

 when the light reaches it. If the velocity of light be V, and 

 that of the earth be v, the line of apertures, giving the apparent 

 direction of the star, must be directed forwards through an angle 

 equal to vlN . More generally, if the angle between the star 

 and the point of the heavens towards which the earth is moving 

 be o, there will be an apparent displacement towards the latter 

 point, expressed by sin a . z^/V, and independent of the position 

 upon the earth's surface where the observation is made. The 

 ratio zz/V is about x^\-^-^. 



The aperture in the anterior screen corresponds to the object- 

 glass of the telescope with which the observation would actually 

 be made, and which is necessary in order to produce agreement 

 of phase of the various elementary waves at a moderately distant 

 focal point. The introduction of a refracting medium would 

 complicate the problem, and is not really necessary for our present 

 purpose. h.%\\d&ht.^x\^o\N-a. {Philosophical Magazine, March 

 1881, " On Images formed without Reflection or Refraction"), 

 the only use of an object-glass is to shorten the focal length. 

 Our imaginary screens may be as far apart as we please, and if 

 the distance is sufficient, the definition, and consequently the 

 accuracy of alignment, is as great as could be attained with the 

 most perfect telescope whose aperture is equal to that in the 

 anterior screen. 



It appears, then, that stellar aberration in itself need present 

 no particular difficulty on the wave theory, unless the 

 hypothesis of a quiescent aether at the earth's surface be re- 

 garded as such. But there are a variety of allied phenomena, 

 mostly of a negative kind, which require consideration before 

 any judgment can be formed as to the degree of success with 

 which the wave theory meets the demands made upon it. In 

 the first place, the question arises whether terrestrial optical 

 phenomena could remain unaffected by the supposed immense 

 relative motion of our instruments and of the aether ; whether 

 reflection, diffraction, and refraction, as ordinarily observed by 

 us, could be independent of the direction of the rays relatively 

 to the earth's motion. It may be stated at once that no such 

 influence has been detected, even in experiments carefully 

 designed with this object in view. 



Another class of experiments, with the results of which theory 

 must be harmonized, are those of Fizeau and Michelson upon 

 the velocity of light in ponderable refracting media which have 

 a rapid motion (relatively to the instruments and other surround- 

 ing bodies) in the direction of propagation, or in the opposite 

 direction. These very important researches have proved that 

 in the case of water the velocity of the ponderable medium is 

 not without effect ; but that the increment or decrement of the 

 velocity of propagation is very decidedly less than the velocity 

 of the water. On the other hand, the motion of air, even at 

 high velocities, has no perceptible influence upon the propagation 

 of light through it. 



Again, it has been found by Airy,' as the result of an experi- 

 ment originally suggested by Boscovitch, that the constant of 

 stellar aberration is the same, whether determined by means of 

 a telescope of the ordinary kind, or by one of which the tube is 

 filled with water. It is clear that, according to Fresnel's views 

 • Pror. Roy. Soc, xx., 1872, p. 35; xxi., 1873, p. 121. 



