STELLAR LIGHT. 89 



to such views because it will be necessary in the sequel that 

 we should consider certain peculiarities ©f motion ascribed 

 to Procyon, which appeared to indicate a disturbance from 

 dark cosmical bodies. It is the object of the present portion 

 of this work to notice the different dir3ctions to which scien- 

 tific inquiry had inclined at the period of its composition and 

 publication, and thus to indicate the individual character 

 of an epoch in the sidereal as well as the telluric sphere. 



The photometric relations (relations of brightness) of the 

 self-luminous bodies with which the regions of space are 

 filled, have for more than two thousand years been an ob- 

 ject of scientific observation and inquiry. The description 

 of the starry firmament did not only embrace determinations 

 of places, the relative distances of luminous cosmical bodies 

 from one another and from the circles depending on the ap- 

 parent course of the sun and on the diurnal movement of 

 the vault of heaven, but it also considered the relative in- 

 tensity of the light of the stars. The earliest attention of 

 mankind was undoubtedly directed to this latter point, in- 

 dividual stars having received names before they were ar- 

 ranged with others into groups and constellations. Among 

 the wild tribes inhabiting the densely- wooded regions of the 

 Upper Orinoco and the Atabapo, where, from the impene- 

 trable nature of the vegetation, I could only observe high 

 culminating stars for determinations of latitude, I frequently 

 found that certain individuals, more especially old men, had 

 designations for Canopus, Achernar, the feet of the Centaur, 

 and a in the Southern Cross. If the catalogue of the con- 

 stellations known as the Catasterisms of Eratosthenes can 

 lay claim to the great antiquity so long ascribed to it (between 

 Autolycus of Pitane and Timocharis, and therefore nearly a 



■would destroy the amount of motion emitted from the luminous mole- 

 cule, so that it would be invisible at great distances." If, with Sir 

 William Herschel, we ascribe to Arcturus an apparent diameter of 0"*1, 

 it follows that the true diameter of this star is only eleven times greater 

 than that of our sun. (Cosmos, vol. i., p. 148.) From the above con- 

 siderations on one of the causes of non-luminosity, the velocity of light 

 must be very different in cosmical bodies of different dimensions. This 

 has, however, by no means been confirmed by the observations hitherto 

 made. Arago says in the Comptes Rendus, t. viii., p. 326, " Les expe- 

 riences sur l'egale deviation prismatique des etoiles, vers lesquelles la 

 terra marche ou dont elle s'eloigne, rend compte de l'egalite de vitesse 

 apparente de toutes le3 etoiles." "Experiments made on the equal 

 prismatic deviation of the stars toward which the earth is moving, and 

 from which it is receding, explain the apparent equality of velocity in 

 the rays of all the stars." 



