RAYS OF THE STARS. 12S 



witli a needle in a card, and I have myself frequently oL- 

 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 why 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- 

 nifyin^ power, giving the smaller disk."! Occult ations 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 i^ 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 Yfill- 

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

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

 minished in a dense mist tliat 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 shipi 

 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 tha 

 multiplied images. Here we have a phenomenon of diffracfioiu oh* 

 B3rved through tine slits. 



t Outlines, ^ 8U). Arago has caused the spurious diameter of Aide- 

 baran to increase from 4" to 15" in the instrument by diminisb.ing tlm 

 ol»ject-glass. 



t Delambre, Hist de V Astr. Moderne, torn, i., p. 193; Aragii, Aunv^ 

 aire, 184-2 p. SfiC. 



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