102 COSMOS. 



coloured rays into white light. For this reason we nost 

 rarely meet with traces of scintillation in Jupiter and Saturn, 

 but more frequently in Mercury and Venus, for the apparent 

 diameters of the discs of these last named planets diminish to 

 4'' - 4 and 9"'5. The diameter of Mars may also decrease to 

 3"*3 at its conjunction. 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 6th and 7th 

 magnitude flickering in various directions, we are led to 

 imagine that we perceive 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 visible to the naked eye ! It was known 

 in ancient times by the Greek astronomers, that the flickering 

 of their light distinguished the fixed stars from the planets ; 

 but Aristotle, in accordance with the emanation and tan- 

 gential theory of vision, to which he adhered, singularly 

 enough ascribes the scintillation of the fixed stars merely 

 to a straining of the eye. "The rivetted stars (the fixed 

 stars)," says he,* 8 " sparkle, but not the planets : for the 

 latter are so near, that the eye is able to reach them ; but 

 in looking at the fixed stars (irpos 8e TOVS pevovras] the eye 

 acquires a tremulous motion owing to the distance and the 

 effort." 



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

 remarkable for great celestial events, when three stars 48 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 

 *o scintillation as the probable criterion of the non-planetary 



a Aristot. de Calo, ii. 8, p. 290, Bekker. 

 43 Cosmos, vol. ii. p. 709. 



