264 optics. 



like radii from a centre. If the sun's rays fall 

 upon a plain mirror, they will continue parallel to 

 each other after reflection. This may be familiarly 

 illustrated by holding a piece of looking-glass in 

 the sunshine, and throwing the reflection on a 

 ceiling. 



As it is by means of the rays that emanate from 

 a body that it is rendered visible, so the eye refers 

 the body itself to the place which is in the di- 

 rection of the rays while entering the eye. But if 

 the rays from the body have suffered any change 

 in their direction, either by refraction or reflection, 

 we shall then have an erroneous conception of the 

 true situation of the body, for we shall refer it to 

 the last direction of the ray. 



In a plane mirror, the image of an object ap- 

 pears in the direction which the rays have, after 

 being reflected from the glass. Thus, if an object 

 be placed at A (Plate 26. fig. 3.), the rays from it 

 which enter the eye C have been reflected in the 

 direction B C; the eye at C, therefore, will refer 

 the situation of the object to the direction C Bj 

 and it will appear to be behind the mirror at D, 

 and just as far from it D E, as it is in reality before 

 it E A. An eye placed at F will see itself as if at 

 G, as far behind the glass as it is actually 

 before it. 



Objects appear larger or smaller to the eye, as 

 the rays which proceed from their extremities to 

 the eye make a greater or a smaller angle at the 

 eye. Thus, the tree A B (Plate 26. fig. 4.) will 

 appear larger than the tree C D, although they are 

 actually of the same size, because the angle A E D 

 is greater than C E D. This will be easily under- 

 stood, because the rays crossing each other as they 

 enter the pupil, the image of A B upon the retina 



