109 



pole, when it passes nearest its zenith under the me 

 ridian, and when it appears on the horizon. This last, 

 he says, will be its smallest distance. He then makes 

 it appear, that refraction is the cause of this pheno- 

 menon. Yet Alhazen advances nothing but what he 

 derived from Ptolemy ; and neither one nor other of 

 them have applied this important discovery in astro- 

 nomy, so as ta deduce from it, that the apparent ele- 

 vation of the stars, when near the horizon, uecessariiy 

 requires to be corrected* 



6. Roger Bacon enquiring into the cause of that 

 difference of magnitude in stars when seen on the ho- 

 rizon, 'from what they have when viewed over head, 

 says, in the first place, that it may proceed from this, 

 u That the rays coming from the star are made to di- 

 verge from eax h other, not only by passing from the 

 rare medium of ether into the denser one of our sur- 

 rounding air, but also by the interposition of clouds 

 and vapot.-S- arising out of the earth, which repeat the 

 retraction and augment the dispersion of the rays, 

 whereby the object must needs be magnified to our 

 eye. Though," says he afterwards, u there has been 

 assigned by Ptolemy and Alhazen another cause for. 

 this; these authors thought that the reason of a star's 

 appearing larger at its rising or setting than when 

 viewed over head, arose from this, that when the star 

 is over head, there are no immediate objects perceived 

 between it and us, so that we judge it nearer to us, 

 and are not surprised at its littleness ; but when a star 

 is nie\ie.d >u the horizon,, it lies then so low, that all 

 we can see upon earih interposes between it and us, 

 which making it appear at a greater distance, we ima- 

 gine it larger than if is. For the same reason the sun 

 and moon, when appearing upon the hor.iz.ou, seciu to 

 be at a greater distance, by reason of the interposition 

 of those objects which are upon the surface of our 

 earth, than when they are over luad ; and consequent- 

 ly there will arise in our minds an idea of their large- 

 ness, augmented by that of their distance, and this of 



