REFLECTED AND TRANSMITTED LIGHT. 



represents a small hole in a shutter admitting a ray of 

 light from the sun into a, darkened room. The light falls 

 upon a mirror lying horizontally upon a table, making with 

 the perpendicular, A D, the angle of incidence, C D A. The 

 ray is reflected in the direction D B, as far to the right of 

 the perpendicular as it came in on the left ; in other words, 

 making the angle of reflection, A D B, equal to the angle 

 of incidence, ADC. 



An instrument has been devised for showing that these 

 angles are exactly equal, so far as mathematical principle 

 of this kind can be shown by experiment. The next en- 

 graving represents this instrument. 



It consists of a graduated circle, in the centre of which 

 the mirror is placed, as shown at n. 

 Attached to the graduated circle are 

 two slender tubes, A and B, made 

 movable upon the arc. The distance 

 of each from the central point above 

 can be easily determined by the grad- 

 uation. It is found, in experimenting 

 with this instrument, that when one 

 of the tubes, A, is so adjusted on the 

 arc upon one side, and the instrument 

 is so placed upon the table that a ray 

 of light from the sun passes through the tube to the mir- 

 ror, and the other tube is placed at the same distance on 

 the other side, then, and only then, will the ray, after re- 

 flection, pass out through the other tube, B. 



In the same manner, when the two tubes are placed at 

 the same distance from the upper middle point of the arc, 

 no matter what the distance is, a person looking through 

 one of them will see any small object a key, for example, 

 held at the opening of the other, showing that in all cases 

 the angles of incidence and reflection are equal. 



MODE OF MEASUREMENT. 



