THE COLOUR OF THE STAKS. 175 



of the telescope, respectively ascribed to Sirius 44 a diameter 

 of 4' and of 2' 20". 



The alternating light and dark rings which surround the 

 small spurious discs of the stars when magnified two -or 

 three hundred times, and which appear iridescent when 

 seen through diaphragms of different form, are likewise the 

 result of interference and diffraction, as we learn from the 

 observations of Arago and Airy. The smallest objects which 

 can be distinctly seen in the telescope as luminous points, 

 may be employed as a test of the perfection in construc- 

 tion and illuminating power of optical instruments, whether 

 refractors or reflectors. Amongst these we may reckon mul- 

 tiple stars, such as c Lyra3, and the 5th and 6th star discovered 

 by Struve,in 1826, and by Sir John Herschel in 1832, in the 

 trapezium of the great nebula of Orion, 48 forming the qua- 

 druple star of that constellation. 



A difference of colour in the proper light of the fixed stars, 



44 Delambre, Hist, de VAstr. moderne, torn. i. p. 193 ; 

 Arago, Annuaire, 1842, p. 366. 



45 " Two excessively minute, and very close companions, to 

 perceive both of which, is one of the severest tests which can be 

 applied to a telescope." (Outlines, 837. Compare also Sir 

 John Herschel, Observations at the Cape, p. 29 ; and Arago, 

 in the Annuaire pour 1834, pp. 302-305.) Among the dif- 

 ferent planetary cosmical bodies by which the illuminating 

 power of a strongly magnifying optical instrument may be 

 tested, we may mention the 1st and 4th satellites of Uranus, re- 

 discovered by Lasselland Otto Struve in 1847, the two inner- 

 most and the 7th satellite of Saturn (Mimas, Enceladus, and 

 Bond's Hyperion), andNeptune's satellite discovered by Lassell. 

 The power of penetrating into celestial space occasioned 

 Bacon, in an eloquent passage in praise of Galileo, to whom 

 he erroneously ascribes the invention of telescopes, to com- 

 pare these instruments to ships which carry men upon an 

 unknown ocean: " Ut propriora exercere possint cum cceles- 

 tibus commercia." (Works of Francis Bacon, 1740, vol. i. 

 Novum Organum, p. 361.) 



