ORGANS 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 Che- 
todon rostratus, 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 object ; then the body being put in an oblique 
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