130 Lord Rayleigh on the 



whoin we owe also a leading feature of the true theory, that 

 is, the explanation of the chromatic effects by reference to the 

 different paths pursued by rays of different colours in virtue 

 of regular atmospheric dispersion. The path of the violet ray 

 lies higher than that of the red ray which reaches the eye of 

 the observer from the same star, and the separation may be 

 sufficient to allow the one to escape the influence of an atmo- 

 spheric irregularity which operates upon the other. In 

 Montigny's view the diversion of the light is caused by total 

 reflexion at strata of varying density. 



But the most important work upon this subject is un- 

 doubtedly that of Eespighi *, who, following in the steps of 

 Montigny and Wolf, applied the spectroscope to the investi- 

 gation of stellar scintillation. The results of these observa- 

 tions are summed up under thirteen heads, which it will be 

 convenient to give almost at full length. 



(I.) In spectra of stars near the horizon we may observe 

 dark or bright bands, transversal or perpendicular to the 

 length of the spectrum, which more or less quickly travel 

 from the red to the violet or from the violet to the red, or 

 oscillate from one to the other colour ; and this however the 

 spectrum may be directed from the horizontal to the vertical. 

 (II.) In normal atmospheric conditions the motion of the 

 bands proceeds regularly from red to violet for stars in the 

 west, and from violet to red for stars in the east ; while in the 

 neighbourhood of the meridian the movement is usually oscil- 

 latory, or even limited to one part of the spectrum. 



(III.) In observing the horizontal spectra of stars more 

 and more elevated above the horizon, the bands are seen 

 sensibly parallel to one another, but more or less inclined 

 to the axis of the spectrum, passing from red to violet or 

 reversely according as the star is in the west or the east. 



(IV.) The inclination of the bands, or the angle formed by 

 them with the axis (? transversal) of the spectrum depends 

 upon the height of the star ; it reduces to 0° at the horizon 

 and increases rapidly with the altitude so as to reach 90° at 

 an elevation of 30° or 40°, so that at this elevation the bands 

 become longitudinal. 



(Y.) The inclination of the bands, reckoned downwards, is 

 towards the more refrangible end of the spectrum. 



(VI.) The bands are most marked and distinct when the 

 altitude of the star is least. At an altitude of more than 40° 

 the longitudinal bands are reduced to mere shaded streaks, and 



* Roma, Atti Nuovi Lincei, xxl (1868) ; Assoc. Francaise, Compt. 

 Rend. i. (1872) p. 169. 



