76 COSMOS. 



are once excited, they retain the impression of hght which 

 they have received, so that the disappearance, obscuration 

 and change of color in a star are not perceived by us to their 

 full extent. The phenomenon of scintillation is more striking- 

 ly manifested in the telescope when the instrument is shaken, 

 for then different points of the retina are successively excited, 

 and colored and frequently interrupted rings are seen. The 

 principle of interference explains how the momentary colored 

 effulgence of a star may be followed by its equally instanta- 

 neous disappearance or sudden obscuration, in an atmosphere 

 composed of ever-changing strata of different temperatures, 

 moisture, and density. The undulatory theory teaches us 

 generally that two rays of light (two systems of waves) em- 

 anating from one source (one center of commotion), destroy 

 each other by inequality of path ; that the light of one ray 

 added to the light of the other produces darkness. When the 

 retardation of one system of waves in reference to the other 

 amounts to an odd number of semi-undulations, both systems 

 endeavor to impart simultaneously to the same molecule of 

 ether equal but opposite velocities, so that the effect of their 

 combination is to produce rest in the molecule, and therefore 

 darkness. In some cases, the refrangibility of the different 

 strata of air intersecting the rays of light exerts a greater in- 

 fluence on the phenomenon than the difference in length of 

 their path.=^ , 



The intensity of scintillations varies considerably in the dif- 

 ferent 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 plan- 

 ets with larger disks is to be ascribed to compensation and to 

 the naturahzing mixture of colors proceeding from different 

 points of the disk. The disk is to be regarded as an aggregate 



by Galileo, Scaliger, Kepler, Descartes, Hooka, 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 discoveiy of the first laws of interference 

 I'egarded scintillation as an inexplicable phenomenon. The erroneous- 

 ness of the ancient explanation, which supposes that vapors ascend and 

 displace one another, is sufficiently proved by the circumstance that we 

 see scintillations with the naked eye, which presupposes a displace 

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

 4" to 5", and are perhaps owing to chasms or inteiTuptions, and there- 

 fore also to the effect of interference of the rays of light." {Ext-^acfs 

 from Arago's MSS. of 1847.) 



* See Arago, in the Annuaire pour 1835 p. 168. 



