110 SPECIAL RESULTS IN THE URANOLOGICAL PORTION 



the heavens with the naked eye, would appear to me a pri- 

 vation. Illusion of the senses, and indistinctness of vision, 

 may perhaps augment the magnificence of the shining canopy 

 of heaven. Arago long ago proposed the question Why is 

 it that, notwithstanding the strong light of the fixed stars, we 

 do not perceive them when rising above the horizon, whilst 

 yet we see the extreme margin of the moon's disc under 

 similar circumstances ? ( 208 ) 



Even the most perfect optical instruments, with the highest 

 magnifying powers, give to the fixed stars false diameters 

 (spurious discs, diametres factices), which, according to Sir 

 John HerscheFs remark, ( 209 ) "with equal magnifying powers 

 diminish as the aperture of the telescope increases." Occul- 

 tations of stars by the Moon's disc show that immersion and 

 emersion are sensibly instantaneous, so much so, that no 

 fraction of a second can be assigned for the time occupied 

 in disappearance or reappearance, The frequent observation 

 of stars in their immersion adhering to the disc of the moon, 

 is a phenomenon of the inflection of light which has no con- 

 nection with the question of the star's diameter. I have 

 already noticed elsewhere, that Sir William Herschel, with a 

 magnifying power of 6500, still found the diameter of 

 a Lyrse 0"'36. The image of Arcturus was so lessened in a 

 thick mist as to be even below 0"'2. It is remarkable that, from 

 the illusion produced by irradiation, Kepler and Tycho 

 Brahe, before the invention of telescopes, ascribed to Sirius a 

 diameter, the one of 4', the other of 2' 20". ( 21 ) The alter- 

 nating light and dark rings which surround the factitious 

 discs of stars, viewed with magnifying powers of two or three 

 hundred, and which, when diaphragms of different shapes are 

 applied, show prismatic colours, are the consequences at once 



