DISCOVERY OF THE LAW OF REFRACTION. 377 



truth of this ; and that the quantities of refraction 

 differ according to the magnitudes of the angles 

 which the directions of the incidental rays (primce 

 linece) make with the perpendiculars to the surface; 

 but he also says distinctly and decidedly that the 

 angles of refraction do not follow the proportion 

 of the angles of incidence (DA). 



This was an important remark ; and if it had 

 been steadily kept in mind, the next thing to be 

 done with regard to refraction was to go on ex- 

 perimenting and conjecturing till the true law of 

 refraction was discovered ; and in the mean time to 

 apply the principle as far as it was known. Alhazen, 

 though he gives directions for making experimental 

 measures of refraction, does not give any Table 

 of the results of such experiments, as Ptolemy had 

 done. Vitello, a Pole, who in the thirteenth cen- 

 tury published an extensive work upon Optics, does 

 give such a table ; and asserts it to be deduced 

 from experiment, as I have already said, (vol. i. 

 p. 117). But this assertion is still liable to doubt 

 in consequence of the table containing impossible 

 observations (E A). 



The principle that a ray refracted in glass or 

 water is turned towards the perpendicular, without 

 knowing the exact law of refraction, enabled ma- 

 thematicians to trace the effects of transparent 

 bodies in various cases. Thus in Roger Bacon's 

 works we find a tolerably distinct explanation of 

 the effect of a convex glass ; and in the work of 



