RAYS or TllK STARS. 129 



with a needle in a card, and I have myself frequently ob- 

 served both Canopus and Sirius in this manner. The same 

 thing occurs in telescopic vision through powerful instru- 

 ments, when the stars appear either as intensely luminous 

 points, or as exceedingly small disks. Although the fainter 

 scintillation of the fixed stars in the tropics conveys a cer- 

 tain impression of repose, a total absence of stellar radiation 

 would, in my opinion, impart a desolate aspect to the firma- 

 ment, as seen by the naked eye. Illusion of the senses, op- 

 tical illusion, and indistinct vision, probably tend to augment 

 the splendor of the luminous canopy of heaven. Arago long 

 since proposed the question w^hy fixed stars of the first mag- 

 nitude, notwithstanding their great intensity of light, can 

 not be seen when rising above the horizon in the same man- 

 ner as under similar circumstances we see the outer margin 

 of the moon's disk.* 



Even the most perfect optical instruments, and those hav- 

 ing the highest magnifying powers, give to the fixed stars 

 spurious disks (diametres factices) ; " the greater aperture," 

 according to Sir John Herschel, " even with the same mag- 

 nifying power, giving the smaller disk."t Occultations of 

 the stars by the moon's disk show that the period occupied 

 in the immersion and emersion is so transient that it can not 

 be estimated at a fraction of a second of time. The frequent 

 occurrence of the so-called adhesion of the immersed star to 

 the moon's disk is a phenomenon depending on inflection of 

 light in no way connected with the question of the spurious 

 diameter of the star. We have already seen that Sir Will- 

 iam Herschel, with a magnifying power of 6500, found the 

 diameter of Vega 0"'36. The image of Arcturus was so di- 

 minished in a dense mist that the disk was below 0""2. It 

 is worthy of notice that, in consequence of the illusion occa- 

 sioned by stellar radiation, Kepler and Tycho, before the in- 

 vention of the telescope, respectively ascribed to Sirius$ a 

 diameter of 4' and of 2' 20". 



* I found an opinion prevalent among the sailors of the Spanish ships 

 of the Pacific, that the age of the moon might be determined before the 

 first quarter by looking at it through a piece of silk and counting the 

 multiplied images. Here we have a phenomenon of diffraction ob- 

 served through fine slits. 



t Outlines, $ 816. Arago has caused the spurious diameter of Alde- 

 baran to increase from 4" to 15" in the instrument by diminishing the 

 object-glass. 



X Delambre, Hist, de VAstr. Modeme, tom. i., p. 193 ; Arago, Annu- 

 aire, 1842, p. 366. 



F 2 



