LIGHT. 21 



The surface of the water furnishes a point of transi- 

 tion from a denser to a rarer medium, and the direction 

 of the ray is thereby changed in accordance with the 

 law above stated. It is thereby enabled to turn a cor- 

 ner, as it were, and come to the eye. 



31. TRIANGULAR PRISM. Bearing the 



What effect ^ 



has a prism on rules last given in mind, it will be readily 



arayof light? 



ing through a prism must be such as is 

 represented in the figure. The ray may 

 be supposed to start from below or above 

 the prism. The line of its passage through 

 the glass will be the same in either case. 



32. Let us suppose it to pass upward 



Mustrate the f rom a bit Q f white paper Qr other ob j ect to 



the eye of an observer above. The appa- 

 rent place of the object will be changed. It will be seen 

 still beneath the prism, not where it actually is, but in 

 the direction in which the ray points as it enters the 

 eye. This experiment may be made equally well 

 with the water prism described in the next paragraph. 



33. CONSTRUCTION or A PRISM. 



How may a 



prism be con- The prism commonly used in 

 optical experiments is of solid 

 glass. In lack of this, its place may be rea- 

 dily supplied by the water prism represented in the fig- 

 ure. A strip of window glass is to be scratched with a 

 file and broken into three pieces of equal length. These 

 are set up, as represented in the figure upon another bit 

 of glass previously warmed, and thickly covered with 

 sealing wax. When the wax is cooled, and the bits 



