230 FERGUSON'S LECTURES. 



LECT. To prove this by experiment ; set the empty vessel 



VII. 



A B C D into any place where the sun shines obliquely 

 and observe the part where the shadow of the edge B C 

 falls on the bottom of the vessel at E ; then fill the ves- 

 sel with water, and the shadow will reach no farther 

 thane; which shews, that the ray a B E, which came 

 straight in the open air, just over the edge of the vessel 

 at B to its bottom at E, is refracted by falling obliquely 

 Refracted on the surface of the water at B ; and instead of going on 

 light j n fljg rec tilineal direction a B E, it is bent downward 

 in the water from B to e; the whole bend being at the 

 surface of the water : and so of all the other rays a b c. 



If a stick be laid over the vessel, and the sun's rays be 

 reflected from a glass perpendicularly into the vessel, 

 the shadow of the stick will fall upon the same part of 

 the bottom, whether the vessel be empty or full ; which 

 shews that the rays of light are not refracted when they 

 fall perpendicularly on the surface of any medium. 



The rays of light are as much refracted by passing 

 out of water into air, as by passing out of air into water. 

 Thus, if a ray of light flows from the point e, under 

 water, in the direction e B ; when it comes to the surface 

 of the water at B } it will not go on thence in the rectili- 

 neal course B d, but will be refracted into the line B a. 

 Therefore, 



To an eye at e looking through a plane glass in the 

 bottom of the empty vessel, the point a cannot be seen, 

 because the side B c of the vessel interposes ; and the 

 point d will just be seen over the edge of the vessel at 



