174 COSMOS. 



the question, why fixed stars of the first magnitude, notwith- 

 standing their great intensity of light, cannot be seen when 

 rising above the horizon in the same manner as under similar 

 circumstances we see the outer margin of the moon's disc. 48 



Even the most perfect optical instruments, and those hav- 

 ing the highest magnifying powers, give to the fixed stars 

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

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

 nifying power giving the smaller disc." 43 Occultations of the 

 stars by the moon's disc show that the period occupied in 

 the immersion and emersion is so transient that it cannot be 

 estimated at a fraction of a second of time. The frequent oc- 

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

 moon's disc, is a phenomenon depending on inflection of light 

 in no way connected with the question of the spurious dia- 

 meter of the star. We have already seen that Sir William 

 Herschel, with a magnifying power of 6500, found the diame- 

 ter of Vega O ff> 36. The image of Arcturus was so dimin- 

 ished in a dense mist, that the disc was below 0"-2. It is 

 worthy of notice that, in consequence of the illusion occasioned 

 by stellar radiation, Kepler and- Tycho, before the invention 



1835, cap. 13, p. 20. The learned editor notices, how- 

 ever, in refutation of Jomard's assertion (Descr. de V Egypte, 

 torn. vii. p. 423), that a star, as the numerical hieroglyphic 

 for 5, has not yet been discovered on any monument or 

 papyrus-roll. (Horap., p. 194.) 



43 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 observed through 

 fine slits. 



48 Outlines, 816. Arago has caused the spurious dia- 

 meter of Aldebaran to increase from 4" to 15" in the instru- 

 ment by diminishing the object-glass. 



