OPTICAL IMAGES. 



Fig. 4. 



reflection already explained. If B D be at right angles to M N, 

 the angle, D B o, will be equal to B a N, and also to D B A, 

 and consequently to B A N, from whence it follows that B A 



is equal to B a, and A N to 

 a N : and since the same 

 will be true of all rays 

 which issue from A towards 

 the reflector, it follows 

 that, after reflection, all 

 such rays will enter the 

 eye, o, as if they had di- 

 verged from . 



The eye o will therefore 

 see the point A in the re- 

 flector as if it were at a. 



7. But since the same 

 will be true of each point 

 in an object, A B (fig. 5), 

 placed before the reflector, 

 it follows that the rays 

 which proceed from the 



several points of the object will, after reflection, enter the eye, as 

 if they came from corresponding points of a similar object a 6, 

 placed just as far behind the reflector 

 as the object itself A B is before it. 



It is evident that in this case the 

 image a b is not only similar to the 

 object but precisely equal to it. Its 

 position relatively to the reflector is 

 similar to that of the object, but in 

 an absolute sense it is different, as 

 will be evident from observing that 

 while the arrow A B points to the left, 

 its image a b points to the right. 



8. It will be perceived, that the 

 reflected rays by which the perception 

 of the image is produced, do not 

 actually form the image. They enter 

 the eye as if they actually came from 

 the several points of such an image 

 as the eye sees, but they do not come from such points. In 

 such cases, where the image is perceived, but not actually pro- 

 duced, it is called a virtual or imaginary image. When the rays 

 by which the image is perceived do actually diverge from the 

 points of the image, the image is said to be HEAL. 

 86 



Fig. 5. 



