Microscopical Essays. 35 



With the center C, and any diftance, let a circle be defcribed. 

 Then let AC be a ray of light, falling upon the denfe medium. 

 This ray, if nothing prevented, would go forward to L 5 but be- 

 caufe the medium Y is fuppofed to be denfer than Z, it will be 

 bent downward toward the perpendicular I K, and defcribe the 

 line C B. 



The ray AC is called the incident ray; and the ray C B, 

 the refracted ray. 



The angle ACI is called the angle of incidence, and the 

 angle BCK is called the angle of refraction. 



If from the point A, upon the right line C I, there be let fall 

 the perpendicular A D, that line is called the fine of the angle of . 

 incidence. 



In the fame manner, if from the point B, upon the right line 

 I K, there be let fall the perpendicular B E, that line will be the 

 fine of the angle of refraction. 



The fines of the angles are the meafures of the refractions, and 

 this meafure is conftant ; that is, whatever is the fine of the angle 

 of incidence, it will be in a conftant proportion to the line of the 

 angle of refraction, when the mediums continue the fame. A 

 general idea of refraction may be formed from the following 

 experiments. 



Let A B C D, Fig. 3, Plate I. reprefent a veffel fo placed, with 

 refpecl to the candle E, that the ftiadow of the fide A C may fall 



