OPTICAL IMAGES. 



But if, leaving the direction E i, it issue from the surface at i, 

 it will be refracted from the perpendicular I IT in a direction 

 such as i E. This is a law of refraction to which there is no 

 exception. 



16. Light will enter a transparent body whatever may be the 

 obliquity with which it falls upon it ; but it must be remembered 

 that a certain proportion of it will be reflected. This proportion 

 is very small, when the light strikes the body with very little 

 obliquity, but it increases as the obliquity is increased, and is very 

 considerable at great obliquities. 



17. This will explain a phenomenon which is familiar to every 

 eye. A spectator stationed on the banks of a river or lake, as at 

 s, fig. 10, will see the opposite bank and objects such as o upon 

 it, reflected in the surface of the water, and will see in the same 

 way distant boats or vessels, such as B, reflected, the images being 

 inverted according to what has been already explained (6, 7). 

 But he will not see any reflection of a near object, such as A. In 



Fig. 10. 



the case of distant objects, such as o and B, the rays o E, B E, 

 which proceed from them striking the surface of the water very 

 obliquely, the part of the light which is reflected in the direction 

 E s is so considerable as to make a very sensible impression on the 

 eye, although it is far from being as strong as a more complete 

 reflection would produce, as is proved by the fact of which every 

 one is conscious, that the images of objects thus reflected in water 

 are far less intense and vivid than images would be reflected from 

 the surface of a looking-glass. 



As for objects, such as A, placed near the spectator, they are 

 not seen reflected, because the rays A E', which proceed from them, 

 strike the water with but little obliquity, and consequently the 

 part of their light which is reflected in the direction E' s towards 

 the spectator is not sufticiently considerable to produce a sensible 

 impression on the eye. 



For this reason, also, a person on board a vessel may see 

 90 



