SCINTILLATION CF THE STARS. 77 



oi stars which naturally compensate for the light destroyed 

 by interference, and again combine the colored rays into white 

 light. For this reason, we most rarely meet with traces of 

 scintillation in Jupiter and Saturn, but more frequently in 

 Mercury and Venus, for the apparent diameters of the disks 

 of these last-named planets diminish to 4" - 4 and 9"-5. The 

 diameter of Mars may also decrease to 3" - 3 at its conjunc- 

 tion. In the serene cold winter nights of the temperate zone, 

 the scintillation increases the magnificent impression produced 

 by the starry heavens, and the more so from the circumstance 

 that, seeing stars of the sixth and seventh magnitude flicker- 

 ing in various directions, we are led to imagine that we per- 

 ceive more luminous points than the unaided eye is actually 

 capable of distinguishing. Hence the popular surprise at the 

 few thousand stars which accurate catalogues indicate as vis- 

 ible to the naked eye ! It was known in ancient times by 

 the Greek astronomers that the flickering of their light dis- 

 tinguished the fixed stars from the planets ; but Aristotle, in 

 accordance with the emanation and tangential theory of vi- 

 sion, to which he adhered, singularly enough ascribes the scin- 

 tillation of the fixed stars merely to a straining of the eye. 

 " The riveted stars (the fixed stars)," says he,* " sparkle, but 

 not the planets ; for the latter are so near that the eye is able 

 to reach them ; but m looking at the fixed stars (npog 6e rovg 

 fievovTag), the eye acquires a tremulous motion, owing to the 

 distance and the efibrt." 



In the time of Galileo, between 1572 and 1604 an epoch 

 remarkable for great celestial events, when three starsf of 

 greater brightness than stars of the first magnitude suddenly 

 appeared, one of which, in Cygnus, remained luminous for 

 twenty-one years Kepler's attention was specially directed 

 to scintillation as the probable criterion of the non-planetary 

 nature of a celestial body. Although well versed in the sci- 

 ence of optics, in its then imperfect state, he was unable to 

 rise above the received notion of moving vapors 4 In. the 

 Chinese Records of the newly appeared stars, according to 

 the great collection of Ma-tuan-lin, their strong scintillation 

 is occasionally mentioned. 



The more equal mixture of the atmospheric strata, in and 

 near the tropics, and the faintness or total absence of scintil- 



* Aristot., De Ccelo, ii., 8, p. 290, Bekker. 

 t Cosmos, vol. ii., p. 326. 



t Causte scintillalionis, in Kepler, De Stella nova in pede Serpentam, 

 1606, cap. xviii., p. 92-97. 



