Theory of Stellar Scintillation. 131 



often can only be observed upon the spectrum as slight 

 general variations of brightness. 



(VII.) As the altitude increases, the movement of the 

 bands becomes quicker and less regular. 



(VIII.) As the prism is turned so as to bring the spectrum 

 from the horizontal to the vertical position, the inclination of 

 the bands to the transversal of the spectrum continually dimi- 

 nishes until it becomes zero when the spectrum is nearly 

 vertical -, but the bands then become less marked, retaining, 

 however, the movement in the direction indicated above (III.) . 



(IX.) Luminous bands are less frequent and less regular 

 than dark bands, and occur well marked only in the spectra 

 of stars near the horizon. 



(X.) In the midst of this general and violent movement of 

 bright and dark masses in the spectra of stars, the black 

 spectral lines proper to the light of each star remain sensibly 

 quiescent or undergo very slight oscillations. 



(XI.) Under abnormal atmospheric conditions the bands 

 are fainter and less regular in shape and movement. 



(XII.) When strong winds prevail the bands are usually 

 rather faint and ill defined, and then the spectrum exhibits 

 mere changes of brightness, even in the case of stars near the 

 horizon. 



(XIII.) Good definition and regular movement of the 

 bands seems to be a sign of the probable continuance of fine 

 weather, and, on the other hand, irregularity in these phe- 

 nomena indicates probable change. 



These results show plainly that the changes of intensity and 

 colour in the images of stars are produced by a momentary 

 real diversion of the luminous rays from the object-glass of 

 the telescope ; that in the neighbourhood of the horizon rays 

 of different colours are affected separately and successively, 

 and that all the rays of a given colour are momentarily with- 

 drawn from the whole of the object-glass. 



Most of his conclusions from observation were readily 

 explained by Bespighi as due to irregular refractions, not 

 necessarily or usually amounting (as Montigny supposed) to 

 total reflexions, taking place at a sufficient distance from the 

 observer. The progress of the bands in one direction along 

 the spectrum (II.) is attributed to the diurnal motion. In 

 the case of a setting star, for instance, the blue rays by which 

 it is seen, pursuing a higher course through the atmosphere, 

 encounter an obstacle somewhat later than do the red rays. 

 Hence the band travels towards the violet end of the spec- 

 trum. In the neighbourhood of the meridian this cause of 

 a progressive movement ceases to operate. 



K2 



