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XI. On the Theory of Stellar Scintillation, 

 By Lord Bayleigh, Sec. R.S.* 



ABAGO'S theory of this phenomenon is still perhaps the 

 most familiar, although I believe it may be regarded 

 as abandoned by the best authorities. According to it the 

 momentary disappearance of the light of the star is due to 

 accidental interference between the rays which pass the two 

 halves of the pupil of the eye or the object-glass of the tele- 

 scope. When the relative retardation amounts to an odd 

 multiple of the half wave-length of any kind of light, such 

 light, it is argued, vanishes from the spectrum of the star. 

 But this theory is based upon a complete misconception. 

 " It is as far as possible from being true that a body emitting 

 homogeneous light would disappear on merely covering half 

 the aperture of vision with a half wave-plate. Such a con- 

 clusion would be in the face of the principle of energy, which 

 teaches plainly that the retardation in question would leave 

 the aggregate brightness unaltered " |. It follows indeed 

 from the principle of interference that there will be darkness 

 at the precise point which before the introduction of the 

 half wave-plate formed the centre of the image, but the light 

 missing there is to be found in a slightly displaced position J. 

 The older view that scintillation is due to the actual diver- 

 sion of light from the aperture of vision by atmospheric 

 irregularities was powerfully supported by Montigny§, to 



* Communicated by the Author. 



t Enc. Brit., " Wave Theory, ' p. 441. 



% Since the remarks in the text were written I have read the version 

 of Arago's theory given by Mascart (Traitt d'Optique, t. hi. p. 348). 

 From this some of the most objectionable features have been eliminated. 

 But there can be no doubt as to Arago's meaning. " Supposons que les 

 rayons qui tombent a gauche du centre de l'objectif aient rencontre, 

 depuis les limites superieures de l'atmosphere, des couches qui, a cause de 

 leur densite, de leur temperature, ou de leur etat hygrometrique, etaient 

 douees d'une refringence differente de celle que possedaient les couches 

 traversers par les rayons de droite j il pourra arriver, qu'a raison de cette 

 difference de refringence, les rayons rouges de droite detruisent en totalite 

 les rayons rouges de gauche, et que le foyer passe du blanc, son 6tat 



normal, au vert ; Voila done le resultat theorique parf aitement 



d'accord avec les observations; voila le phenomene de la scintillation 

 dans une lunette rattache d'une maniere intime a la doctrine des inter- 

 ferences" (V Annuaire du Bureau des Longitudes pour 1852, pp. 423, 425). 



That the difference between Arago's theory and that followed in the 

 present paper is fundamental will be recognized when it is noticed that, 

 according to the former, the colour effects of scintillation would be nearly 

 independent of atmospheric dispersion. Arago gives an interesting sum- 

 mary of the views held by early writers. 



§ Mem. de V Acad. d. Bruxelles, t. xxviii. (1856), 



Phil. Mag. S. 5. Vol. 36. No.-218. July 1893. K 



