108 



planets when they appear on the horizon. This last 

 point, disc issed with so much warmth between Malle- 

 tranche and Regis, had already been adjusted by 

 Ptolemy. 



4. Ptolemy, and after him Alhazen, said, " that 

 when a ray of light passes from a more rare, into a 

 jnore dense medium, it changes its direction when it 

 Arrives upon the surface of the latter, describing a line 

 which intersects the angle made by that of its first di- 

 jection, and a perpendicular falling upon it from the 

 more dense medium." Bacon adds, after Ptolemy, 



;i that the angle formed by the coincidence of those 

 two lines, is not always equally divided by the refract- 

 ed ray ; because in proportion to the greater or less 

 Density of the medium, the ray is more or less refract* 

 ed, or obliged to decline from its first direction." In 

 this he approaches very near to the reason assigned by 

 SIT Isaac' Newton, who deducing the cause ot refrac- 

 tion from the attraction made upon the ray of light 

 by the bodies surrounding it, says, " that mediums 

 are more or kss attractive in proportion to their 

 density ." 



5, Ptolemy, acquainted with the principle of the re- 

 fraction of light, could not fail to conclude that this 

 was the cause also of what was called astronomic re. 

 fraction, or of the appearance of planets upon the 

 horizon before they came there; having recourse there- 

 fore to this principle, he accounted for those appear, 

 ances from the ditlerence there was between the me- 

 dium of air and that of ether which by beyond, it ; so 

 truit the rays of light coming from the planet, and en- 

 tering into the denser medium of our atmosphere, must 

 of course be so attracted as to change tiieir direction, 

 and by that means bring the star to our view, before 

 h really came upon the horizon. Alhaz.n tells us of a 

 method whereby we may assure 01 rselvts of this inuh 

 by observation. He bids us take an armillary sphere, 

 4ud u^onlt measure the distance of any star liom. the, 



