SCINTILLATION OF THE STARS. 101 



the refrangibility of the different strata of air intersecting the 

 rays of light exerts a greater influence on the phenomenon 

 than the difference in length of their path. 41 



The intensity of scintillations varies considerably in the 

 different fixed stars, and does not seem to depend solely on 

 their altitude and apparent magnitude, but also on the nature 

 of their own light. Some, as for instance Vega, flicker less 

 than Arcturus and Procyon. The absence of scintillation in 

 planets with larger discs, is to be ascribed to compensation 

 and to the neutralizing mixture of colours proceeding from 

 different points of the disc. The disc is to be regarded as 

 an aggregate of stars which naturally compensate for the 

 light destroyed by interference, and again combine the 



" It follows from these considerations that scintillation must 

 necessarily be referred to the phenomena of luminous inter- 

 ferences alone. The rays emanating from the stars, after 

 traversing an atmosphere composed of strata having different 

 degrees of heat, density, and humidity, combine in the focus of 

 a lens, where they form images perpetually changing in 

 intensity and colour, that is to say, the images presented by 

 scintillation. There is another form of scintillation, inde- 

 pendent of the focus of the telescope. The explanations of 

 this phenomenon advanced by Galileo, Scaliger, Kepler, Des- 

 cartes, Hooke, Huygens, Newton, and John Michell, which I 

 examined in a memoir presented to the institute in 1840 

 (Comptes Rendus, t. x. p. 83), are inadmissible. Thomas 

 Young, to whom we owe the discovery of the first laws of 

 interference, regarded scintillation as an inexplicable phe- 

 nomenon. The erroneousness of the ancient explanation 

 which supposes that vapours ascend and displace one another, 

 is sufficiently proved by the circumstance that we see scintil- 

 lations with the naked eye, which presupposes a displacement 

 of a minute. The undulations of the margin of the sun are 

 from 4" to 5", and are perhaps owing to chasms or interruptions, 

 and therefore also to the effect of interference of the rays of 

 light." (Extracts from Arago'sMSS. 0/1847.) 



11 See Arago, in the Annuaire pour 1831, p. 168. 



