optics. 269 



on account of the less or greater divergency of 

 the pencils of rays which proceed from the object; 

 for the less they diverge, the sooner they are con- 

 verged to points by reflection; and the more they 

 diverge, the farther they must be reflected before 

 they meet. 



If the object be in the centre of the mirror's 

 concavity, the image and object will be coincident, 

 and equal in bulk. 



If a man place himself directly before a large 

 concave mirror, but farther from it than its centre 

 of concavity, he will see an inverted image of 

 himself in the air, between him and the mirror, of 

 a less size than himself. And if he hold out his 

 hand towards the mirror, the hand of the image 

 will come out towards his hand, and coincide with 

 it, of an equal bulk, when his hand is in the centre 

 of concavity; and he will imagine he may shake 

 hands with his image. If he reach his hand far- 

 ther, the hand of the image will pass by his hand, 

 and come between it and his body; and if he move 

 his hand towards either side, the hand of the 

 image will move towards the other; so that what- 

 ever way the object moves, the image will move 

 the contrary way. 



All the while a bye-stander will see nothing of 

 the image, because none of the reflected rays that 

 form it enter his eyes. 



Let a fire be made in a large room, and a 

 smooth mahogany table be put at a good dis- 

 tance near the wall, before a large concave mirror, 

 so placed that the light of the fire may be reflected 

 from the mirror to its focus upon the table; if a 

 person stand by the table, he will see nothing upon 

 it but a long beam of light; but if he stand at a 

 distance towards the fire, not directly between the 



