Q. _Why does a river always appear more shallow than it really is?_ A. Because the light of the bottom of the river is REFRACTED as it emerges out of the water: and (as a stick is not so long when it is _bent_, as it is when it is _straight_) so the river seems less deep than it really is. Q. _How much deeper is a river than it seems to be?_ A. One-third. If, therefore, a river seems only 4 feet deep, it is really 6 feet deep. N. B. Many boys get out of their depth in bathing, in consequence of this deception. Remember, a river is always one-third deeper than it appears to be:--thus, if a river seems to be 4 feet deep, it is in reality 6 feet deep, and so on. Q. _Why do fishes always seem to be nearer the surface of a river than they really are?_ A. Because the rays of light from the fish are _refracted_ as they emerge from the eye: and (as a bent stick is not so far from end to end as a straight one) so the fishes appear nearer our eye than they really are. Q. _Why are some persons NEAR-SIGHTED?_ A. Because the COR´NEA of their eye is so _prominent_, that the image of distant objects is reflected _before it reaches the_ RET'INA; and, therefore, is not distinctly seen. N.B. The cor´nea shields the CRYSTALLINE LENS, and is more or less convex according to the lens which it covers. Q. _What is meant by the "COR'NEA of the EYE?"_ A. All the _outside_ of the visible part of the _eye-ball_. [Illustration: The curve A B C is called the COR'NEA. If this curve be too prominent (or convex), the eye is near-sighted. If too flat (or concave), the eye is far-sighted.] Q. _What is meant by the "RET'INA of the EYE?"_ A. The net-work which lines the _back of the eye_, is so called. [Illustration: The net-work ABC is called the ret'ina, and the projecting part DEF is called the cor'nea.] Q. _What sort of GLASSES do NEAR-SIGHTED persons wear?_ A. If the cor'nea be _too convex_ (or projecting), the person must wear double _concave glasses_, to counteract it. Q. _What is meant by "DOUBLE CONCAVE GLASSES?"_ A. Glasses hollowed in _on both sides_. [Illustration: The figure A is double concave, or concave on both sides.] Q. _What is meant by the "IMAGE of objects being reflected BEFORE it reaches the RET'INA?"_ A. If the cor'nea be _too convex_, the image of a distant object is reflected (on the vitreous humours of the eye) _before it reaches the ret'ina_. [Illustration: Thus the image is reflected at DE, instead of on ABC (the ret'ina).] Q. _What is the use of DOUBLE CONCAVE SPECTACLE glasses?_ A. Near-sighted spectacles _cast the reflection further back;_ and the image (being _thrown upon the ret'ina_) becomes visible. Q. _Why are OLD people FAR-SIGHTED?_ A. Because the humours of their eyes _are dried up by age_, and the COR'NEA _sinks in_, or becomes flattened. Q. _Why does the FLATTENING of the COR'NEA prevent persons seeing objects which are NEAR?_