of the Heavenly Bodies, 16 



towards the perpendicular which lies in that plane, the ray 

 must continue to move in that plane. Hence the refraction is 

 always in a vertical circle." 



These are the accepted doctrines of the day, and delivered 

 as such by the Professor. 



According to these statements, ** when the light passes (ob- 

 liquely it should have been stated), out of any medium into one 

 of greater density, it is found to deviate." To produce this 

 deviation however, the circumstances stated are not sufficient. 

 The light must pass not only out of " any medium into one of 

 greater density," but out of one medium into another — into 

 another and not the same — not the same even increased in density 

 — into another distinct and separate medium, not merely of 

 greater density but of greater refractive power. Refractive 

 power is not identical with density. The refractive power of 

 glass for instance, is to that of water as 55 to 34, its density 

 as 87 to 34. Change of density alone, and not of medium, and 

 of refractive power will not produce the reflection, refraction, 

 dispersion, or change of direction of light. Thus the principles 

 assumed to account for the apparent changes of place in the 

 heavenly bodies fail, together with all the observations de- 

 pendent upon and connected with them. 



A ray of light always moves after refraction in the plane of 

 incidence, whether it falls on a plane or curved surface. That 

 refracted rays therefore may move in vertical circles only, they 

 must come in vertical planes only. But rays of light do not 

 necessarily move either before or after their supposed refractions 

 in vertical planes, and their bendings therefore are not always 

 necessarily in vertical circles. If this were the case, no changes 

 of the diameters, parallel to the horizon, of sun or moon could 

 ever take place, and all measurements thereof to discover any, 

 would be useless. It is however to be conjectured and feared, 

 that this opinion, of the supposed refractions necessarily being 

 in vertical circles, has influenced observation, has led to erro- 

 neous conclusions respecting the phsenomena, and affecting 

 even the judgment of observers in their measurements and 

 estimates, has contributed to establish and continue error. 



