OF OPTICS. 255 



recedes from the mirror, the image approaches nearer to 

 it ; and, as the object approaches nearer to the mirror, 

 the image recedes farther from it ; on account of the 

 lesser or greater divergency of the pencils of rays which 

 proceed from the object ; for, the less they diverge, the 

 sooner they are converged to points by reflection ; and 

 the more they diverge, the farther they must be reflected 

 before they meet. 



If the radius of the mirror's concavity and the dis- 

 tance of the object from it be known, the distance of 

 the image from the mirror is found by this rule : divide 

 the product of the distance and radius by double the 

 distance made less by the radius, and the quotient is the 

 distance required. 



If the object be iu the center of the mirror's conca- 

 vity, the image and object will be coincident, and equal 

 in bulk. 



If a man places himself directly before a large con- 

 cave mirror, but farther from it than its center of con- 

 cavity, 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 holds 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 center of concavity ; and he will ima- 

 gine he may shake hands with his image. If he reaches 

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

 hand, and come between his hand and his body : and if 

 he moves his hand towards either side, the hand of the 

 image will move towards the other; so that whatever 

 way the object moves, the image will move the con- 

 trary. 87 



Note 67 . A very terrific spectacle has been exhibited, in which a 

 hand grasping a dagger appears to strike those who approach the 

 recess in which the mirror is placed. To produce this effect, the 

 person on whom the deception is intended to be practised is made to 

 approach armed with a polished weapon, and his body being illu- 



