OltGANS OF PERCEPTION. 195 



objects exciting the sensation. Rays reflected from a mir- 

 ror, exhibit to us objects in a direction in which they do 

 not exist ; and a similar deception is successful, when we 

 mistake the shadow for the substance. Rays which are re- 

 fracted in passing through media of different densities, like- 

 wise exhibit objects in the direction in which they do not 

 exist. The rays of light from an object in water, coming 

 obliquely to our eye, in air, are deflected from the perpen- 

 dicular, so that it appears nearer the horizon or higher in 

 the water, than it actually is, as is seen by immersing the 

 lower end of a pole in water. On this principle it is well 

 known, that the bottom of a river appears nearer the sur- 

 face, or the water seems shallower than it really is, a de- 

 ception which has proved fatal to many who have ventured 

 into streams with which they were unacquainted. In at- 

 tempting to kill fish in the water with a spear, this source 

 of error will prove very inconvenient, until corrected by 

 experience. Herons, gulls, soland geese and other animals 

 which dart from the air upon their prey in the water, must 

 often miss their aim in youth, from this optical illusion. 

 When rays of light pass from an object in the air, to an 

 eye in the water, they are bent towards the perpendicular, 

 and the object appears nearer the zenith than it actually is. 

 Trouts and other fish, which occasionally catch insects fly- 

 ing over the surface of the water, must learn to avoid this 

 source of error, or meet with frequent disappointments. 

 There is one fresh water fish, a native of India, the Chte- 

 todon restrains, or Shooting Fish, which is able to cor- 

 rect this error with wonderful dexterity. " When the Ja- 

 culator fish intends to catch a fly or any other insect, 

 which is seen at a distance, it approaches very slowly and 

 cautiously, and comes as much as possible perpendicularly 

 under the obj ect ; then the body being put in an oblique 



