IMAGES BY REFRACTION. 



Fig. 11. 



plainly enough the banks or shores reflected in the water ; but 

 if he lean over the bulwark, and look down, he cannot see his 

 own image. 



18. In general, the illustrations and imagery of poetry, drawn 

 from natural phenomena, are just and true. Yet this is not in- 

 variably the case. Every one will perceive from what has just 

 been stated, that the fable of the Dog and the Shadow, which has 

 been handed down through so many ages, diffused through so 

 many languages, and taught so universally in the nursery and the 

 school, is a most gross optical blunder. 



19. If a visible object be placed below a transparent body, as, 

 for example, at the bottom of a reservoir of water, or attached to 

 the lower surface of a plate of glass, an observer above will see, 

 not the object itself, but an optical image of it, which will be 

 nearer to the surface, or less deep than the object. A reservoir 

 of water, a river, or a lake, or the sea, when not too deep to allow 

 the bottom to be visible, will on this account always appear to be 

 less deep than it really is, 



because the optical image 

 of the bottom, which is in 

 fact what the observer 

 sees, is less deep than the 

 bottom itself. After what 

 has been stated above, this 

 is easily explained. 



Let K (fig. 11) be a point 

 of any object below the 

 surface A c of any transpa- 

 rent body. The rays B, D, 

 which diverge from n, will, 

 after emerging, be deflected from the perpendicular in the direc- 

 tions D E, and will enter the eye of an observer as if they came 

 from i, a point less deep than B. The point E will, therefore, 

 be seen as if it were at I, and the same being true of all the 

 points of the object, it follows that an optical image of the object 

 will be formed at a certain depth below the surface, less than the 

 depth of the object. 



This image will evidently be imaginary, since the rays by which 

 it is produced diverge from the surface of the transparent body, 

 but not from the points of the image. 



The greater the refracting power of the body is the more the 

 rays D E, emerging from the surface, will be deflected from, 

 the perpendicular, and consequently the nearer the point I of 

 their divergence, or, what is the same, the image, will be to the 

 surface. 



91 



