130 COSMOS. 



The alternating light and dark rings which surround the 

 email spurious disks 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 observ- 

 ations 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 construction and 

 illuminating power of optical instruments, whether refractors 

 or reflectors. Among these we may reckon multiple stars, 

 such as e Lyrse, and the fifth and sixth star discovered by 

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

 trapezium of the great nebula of Orion,* forming the quad- 

 ruple star 6 of that constellation. 



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

 as well as in the reflected light of the planets, was recog- 

 nized at a very early period ; but our knowledge of this re- 

 markable phenomenon has been greatly extended by the aid 

 of telescopic vision, more especially since attention has been 

 so especially directed to the double stars. We do not here 

 allude to the change of color which, as already observed, ac- 

 companies scintillation even in the whitest stars, and still 

 less to the transient and generally red color exhibited by 

 stellar light near the horizon (a phenomenon owing to the 

 character of the atmospheric medium through which we see 

 it), but to the white or colored stellar light radiated from 

 each cosmic al body, in consequence of its peculiar luminous 

 process, and the different constitution of its surface. The 

 Greek astronomers were acquainted with red stars only, 

 while modern science has discovered, by the aid of the tele- 



* " Two excessively minute and very close companions, to perceive 

 loth of which is one of the severest tests which can be applied to a tel- 

 escope." (Outlines, 837. Compare also Sir John Herschel, Observ- 

 ations at the Cape, p. 29 ; and Arago, in the Annuaire pour 1834, p. 

 302-305.) Among the different planetary cosmical bodies by which 

 the illuminating power of a strongly magnifying optical instrument may 

 be tested, we may mention the first and fourth satellites of Uranus, re- 

 discovered by Lassell and Otto Struve in 1847, the two innermost and 

 the seventh satellite of Saturn (Mimas, Enceladus, and Bond's Hyperi- 

 on), and Neptune's satellite discovered by Lassell. The power of pen- 

 etrating into celestial space occasioned Bacon, in an eloquent passage 

 in praise of Galileo, to whom he erroneously ascribes the invention of 

 telescopes, to compare these instruments to ships which cany men upon 

 an unknown ocean : " Ut propriora exercere possiut cum ccelestibus 

 commercia." ( Works of Francis Bacon, 1740, vol. i., Novum Orga- 

 num, p. 361.) 



