14 On the apparent Changes 



place in the light divided at the confines of different transparent 

 bodies, similar to that produced in the light divided within 

 doubly refracting crystals. Thus from similarity of effects is 

 established the suggested similarity of causes in both, of causes 

 existent, and derived from the phsenomena. 



These supposed causes of the changes of place in the 

 heavenly bodies being thus disposed of, the modes of esti- 

 mating or calculating their amount may be next considered. 

 Together with these unsupported hypotheses of causes, may 

 at the same time. be disposed of, all methods of calculation 

 founded exclusively on the existence and operation of these 

 causes. 



Professor Vince, in his Complete System of Astronomy, has 

 detailed at large the principles adopted to account for these 

 apparent changes of place, and the different methods invented 

 for estimating their amount, and that of the occasional variations 

 observed in these changes themselves. 



** When a ray of light passes out of a vacuum into any 

 medium^ or out of any medium into one of greater density, it 

 is found to deviate from its rectilinear course towards a per- 

 pendicular to the surface of the medium into which it enters. 

 Hence light passing out of a vacuum into the atmosphere will, 

 where it enters, be bent towards a radius drawn to the earth's 

 centre, the top of the atmosphere being supposed to be spherical 

 and concentric with the centre of the earth ; and as in ap- 

 proaching the earth's surface, the density of the atmosphere 

 continually increases, the. rays of light as they descend are 

 constantly entering into a denser medium, and therefore the 

 course of the rays will continually deviate from a right line 

 and describe a curve ; hence at the surface of the earth the 

 rays of light enter the eye of the spectator in a different direc- 

 tion from what they would have entered, if there had been no 

 atmosphere ; consequently, the apparent place of the body from 

 which the light comes must be different from the true place. 

 Also the refracted ray must move in a plane perpendicular to 

 the surface of the earth ; for conceiving a ray to come in that 

 plane before it is refracted, then the attraction being always 



