A Guide To The Scientific Knowledge Of Things Familiar Part 86

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 s.h.i.+elds 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_.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: The net-work ABC is called the ret'ina, and the projecting part DEF is called the cor'nea.]

Q. _What sort of GLa.s.sES do NEAR-SIGHTED persons wear?_

A. If the cor'nea be _too convex_ (or projecting), the person must wear double _concave gla.s.ses_, to counteract it.

Q. _What is meant by "DOUBLE CONCAVE GLa.s.sES?"_

A. Gla.s.ses hollowed in _on both sides_.

[Ill.u.s.tration: 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_.

[Ill.u.s.tration: Thus the image is reflected at DE, instead of on ABC (the ret'ina).]

Q. _What is the use of DOUBLE CONCAVE SPECTACLE gla.s.ses?_

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?_

A. As the cor'nea _is too flat_, the image of any near object is formed _behind the RET'INA of the eye_, and is not seen at all.

[Ill.u.s.tration: The reflection is made at DE, instead of at ABC (the retina).]

Q. _What sort of GLa.s.sES do OLD people WEAR?_

A. As their cor'nea is _not sufficiently convex_, they must use _double convex gla.s.ses_, to enable them to see objects near at hand.

Q. _What sort of gla.s.ses are DOUBLE CONVEX SPECTACLE-GLa.s.sES?_

A. Gla.s.ses which _curve outwards_ on both sides.

[Ill.u.s.tration: The figure A is double convex, or convex on both sides.]

Q. _What is the USE of DOUBLE CONVEX spectacle-gla.s.ses?_

A. As the image of near objects is reflected _behind the RET'INA_, these double convex gla.s.ses _shorten the focus of the eye_, and bring the image _into the eye_ (upon the ret'ina).

Q. _Why do NEAR-SIGHTED persons bring objects CLOSE to the eye, in order to SEE THEM?_

A. As the distance between the _front and back of their eye is too great_, distant objects are reflected _before they reach the ret'ina_; therefore, near-sighted persons bring the objects _closer_, in order that the reflection _may be cast further back_, (to reach the ret'ina).

Q. _Why do OLD people HOLD objects FURTHER OFF, in order to see them better?_

A. As the distance between the _front and back of their eye is not great enough_, the reflection of near objects is thrown _beyond the ret'ina_; therefore, they hold objects _a long way off_, in order to bring their images _forward_ (so as to cast it on the ret'ina).

Q. _Why are HAWKS able to see such an IMMENSE way off?_

A. Because they have a muscle in the eye which enables them to _flatten their cor'nea_, by drawing back the crystalline lens.