Chap. 4'] Angles of Incidence and Refra^kn. 201 



denfity, that which is of an oily or inflammable nature 

 will have a greater refracting power than the other. 



The angle of refraction depends on the obliquity of 

 the rays falling on the refracting furface being fnch al- 

 ways that the fine of the incident angle is to the fine 

 of the refracted angle in a given proportion. 



The incident angle is the angle made by a ray of 

 light, and a line drawn perpendicular to the refracting 

 furface at the point where the light enters the furface; 

 and the refracted angle is the angle made by the ray 

 in the refracting medium with the fame perpendicular 

 produced. The fine of the angle is a line which ferves 

 to meafure the angle, being drawn from a point in 

 one leg perpendicular to the other. 



In pafling from a rare into a denfe medium, or frora 

 one denfe medium into a denfer medium, a ray of 

 light is refracted towards the perpendicular, that is, fo 

 that the angle of refraction fnall be lefs than the angle 

 of incidence; on the contrary, in pafiing from a 

 denfe medium into a rare medium, or from one rare 

 medium into a rarer, a ray of light is refracted from 

 the perpendicular. Thus, in paffing from empty fpacc 

 into air, or any other medium whatever, the ray is 

 bent towards the perpendicular, and in paffing from 

 any other medium into pure fpace, it is bent the con- 

 trary way, that is, from the perpendicular ; the fame 

 effects will take place in paffing from air into glafs, 

 and from glafs into air, &c. 



To render this perfectly clear, let us have recourfe 

 to Plate VIII. fig. 2. If a ray of light p C paries from 

 air into water, in the direction ^>C, perpendicular to the 

 plane D*/, which feparates the two mediums, it fuffers 

 no refraction, becaufe one of the eflentials is wanting 

 ' to that effect, viz, the obliquity of the incidence. 



But 



