170 Experimental Philosophy. [Lecture 12. 



rays are rendered less convergent, you will easily 

 comprehend why objects are diminished by it. 

 By the rays being made less convergent, the 

 visual angle is diminished; for, you know, we 

 see every object in the line in which the rays of 

 light last approached the eye. By the same rule, 

 a concave mirror magnifies or enlarges the image 

 of an object ; for the visual angle is enlarged or 

 rendered more obtuse, and consequently the image 

 is magnified in proportion to the curvature of the 

 . concave surface. 



To prove what I have just now laid down with 

 respect to convex mirrors, in fig. 59, a b is a 

 dart, which is seen in the convex mirror c d. 

 Now, though rays issue from the object a b in all 

 directions, as was explained in the tenth lecture, 

 Plate XI. fig. 46, yet it is seen only by means of 

 those which are included within the space between 

 o and 7i, because it is only those which can be 

 reflected to the eye at r. Now you will easily 

 perceive that if these rays had gone forward in 

 the direction in which they were proceeding, 

 they would have united at p, and the object 

 would have been seen of its full size. As it is, 

 however, the rays are reflected less convergent 

 than they were in their natural course, and the 

 angle o r n, being less than the angle a p 5, the 

 image at s appears smaller than the object, and 

 nearer to the surface of the mirror. The reason 

 of this last effect has been already explained, 

 when I said that objects are rendered visible, not 



