LECTURE XI. 



EXPERIMENTAL PHILOSOPHY. 



a 



THE REFRANGIBIL1TY OT LIGHT. 



THE natural progress of light, we have already 

 seen, is in straight lines; yet it is found to be 

 subject to the laws of attraction, as well as all 

 other bodies; and, under the impulse of that 

 power, it is sometimes turned out of its direct 

 course. This only happens when it passes out 

 of one medium into another of a different den- 

 sity, as from air into water or glass, or from 

 water or glass into air; and this property of 

 light is called refraction. A very easy experi- 

 ment will show you what is meant by refraction ; 

 for. if you put one end of a straight stick into 

 water, it will appear at the surface as if it were 

 broken, that is, refracted, from the Latin verb 

 refrangv, to break. 



It is evident that this effect can only arise from 

 the rays of light being drawn or attracted out of 

 their direct course ; and this I shall prove by a 

 very common and a very easy experiment. Put 

 a shilling, or any other conspicuous but small 

 object, into a bason or other vessel, and then re- 

 tire to such a distance, as that the edge of the 

 vessel shall just hide it from your sight. If, then, 

 you remain motionless while the vessel is filled with 



