. Animal physiology. Physiology, Comparative; Physiology, Comparative. 398 REFRACTION OF LIGHT. instance), without obstruction. In passing from a single lumi- nous point into space, they diverge or separate, in such a manner as to cover a larger and larger surface as they advance ; and at the same time the intensity of the light is diminished. (See Mechan. Philos. §.91). But when the rays pass from one medium to another more or less dense, they are bent out of their straight course, or refracted ; unless they should happen to pass from the one to the other in a direction perpendicular to the p
. Animal physiology. Physiology, Comparative; Physiology, Comparative. 398 REFRACTION OF LIGHT. instance), without obstruction. In passing from a single lumi- nous point into space, they diverge or separate, in such a manner as to cover a larger and larger surface as they advance ; and at the same time the intensity of the light is diminished. (See Mechan. Philos. §.91). But when the rays pass from one medium to another more or less dense, they are bent out of their straight course, or refracted ; unless they should happen to pass from the one to the other in a direction perpendicular to the plane which separates them. This may be made evident by a very simple experiment. Place a coin or any heavy body («, Fig. 194) at the bottom of a basin, and then retreat from it, until the coin is hidden by the edge of the basin; if water be then poured in, up to the level c, the coin will again become visible, although neither its own place, nor that of fig. 194. the observer, has undergone any change. The reason of this is, that the rays of light, as they pass out of the water, are bent downwards, or from the perpen- dicular ; so that they reach the eye of the observer, when it is situated at a lower point than that which the rays would have reached, if they had proceeded in a straight line. Thus the eye of the observer, situated at the end of the line a c, could not see the coin in a straight line; because rays passing in that line would be interrupted by the opaque sides of the basin ; but it receives the ray which was passing through the water in the direction a d, and which was bent downwards at the moment of quitting it. If the eye had been placed directly over the coin, however, so that the ray passing from the latter to it would have emerged from the water in a direction perpendicular to its sur- face, no change in the apparent place of the object would have been made by pouring in the water ; since a ray that passes from one medium, to another however different, in a
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