294 



SENSE OF SIGHT. 



always in a straight line from the luminous object; and, accordingly, if 

 the course of the rays be modified before they reach the organ, we fall 

 into an optical illusion. .Such modifications arise either from the re- 

 flection or refraction of the rays proceeding from the object that causes 

 the sensation. By reflection of the rays, we experience the illusion 

 caused by mirrors. A ray of light, K C, Fig. 77, falling upon a plane 

 mirror, I J, is reflected back in the same line ; but, as we have seen, the 

 object does not appear to be at K, but at E. Again, a ray of light, 

 proceeding obliquely from B, and impinging on a plane mirror at C, is 

 reflected in the direction of C A; but to the eye at A, the object B 



appears to be at H, in the prolon- 

 Fi s- 13 - gation of the ray that reaches the 



eye. 



ci :"" If 



the 



be 



the 



Concave Mirror. 



Fig. 131, 



mrror e concave, 

 object appears magnified, provided 

 the light from the upper part of 

 the object, as A B, Fig. 130, be 

 reflected to an eye at F, and that 

 from the lower part of the object 

 meet the other at this point. To 

 an eye so placed, the object appears magnified and seems to be at C 

 D, or in the prolongation of the rays which fall upon the cornea. If 

 the mirror be convex (Fig. 131), for like reasons, the cross will seem 

 to be smaller. 



The cornea constitutes a mirror of this class, in which we have an 

 accurate miniature representation of objects. 



Rays that are refracted in passing through 

 different media, give rise to visual illusions. 

 We have seen, that the ray from an object at 

 ^^r , F 5 Fig. 77 ? i n the pool of water, I J, does 

 """*\<C^ not proceed into the air in the direction of 



F C 0, but in that of the line F C A; and if 

 we suppose the eye to be placed at A, the 

 object will not be seen at F, but will appear 

 to be at /; the pool will, consequently, appear 

 shallower than it really is, by the space at 

 which/ is situate above the bottom. We can now understand why 

 rivers appear less deep than they are, when viewed obliquely; and why 

 the lower end of a pole, immersed in water, should, when seen obliquely, 

 appear to be bent towards the surface. In shooting fish in the water, 

 or in attempting to harpoon them, this source of error has to be cor- 

 rected. Birds, too, that live upon the inhabitants of the water, have 

 to learn, from experience, to obviate the optical illusion; or to descend 

 perpendicularly upon their prey, in which direction, as we have seen, 

 no refraction takes place. Similar remarks apply to fish that leap out 

 of the streams to catch objects in the air. The Ohsetodon rostratus, 

 about six or eight inches long, frequents the sea-shores in the East 

 Indies: when it observes a fly sitting on the plants that grow in shallow 

 water it swims to the distance of five or six feet, and then, with sur- 

 prising dexterity, ejects out of its tubular mouth a single drop of water, 





Convex Mirror. 



