A Guide To The Scientific Knowledge Of Things Familiar Part 84

[Ill.u.s.tration: Here of the rays SA, SB, and SC, only the ray SC meets the eye of the spectator D.

The spot C, therefore, will appear luminous to the spectator D, but no other spot of the water ABC.]

Q. _At night the MOON seems to be reflected from only ONE SPOT of a lake of water, while all the REST seems DARK,--WHY is this?_

A. Because the rays (which fall at various degrees of obliquity on the lake) _are reflected at similar angles_; but as only those which _enter the eye of the spectator_ will be visible, all the water will appear dark _but that one spot. (See last figure.)_

Q. _Why are MORE STARS visible from a MOUNTAIN, than from a PLAIN?_

A. As the air _absorbs and diminishes light_, the _higher we ascend_, the _less light will be absorbed_.

Q. _Why does the SUN seem LARGER at his RISE and SET, than it does at NOON?_

A. Because the earth is surrounded by air, which acts like a _magnifying gla.s.s_; and when the sun is near the horizon (as its rays _pa.s.s through more of this air_), it is more magnified.

[Ill.u.s.tration: Here SC represents a ray of the sun at noon, and MC a ray of the sun near the horizon. DEG represents the air or atmosphere around the earth.

Because EC is longer than DC, therefore the rays of the sun at M pa.s.s through _more air_ than the rays of the sun at S, and the sun is more magnified.]

Q. _Why does the RISING and SETTING MOON appear so much LARGER, than after it is risen higher above our heads?_

A. Because the earth is surrounded by air, which acts _like a magnifying gla.s.s_; and when the moon is near the horizon (as its rays pa.s.s through more of this air) it is more magnified. _(See last figure.)_

Q. _When CANDLES are LIGHTED, we CANNOT SEE into the STREET or road,--WHY is this?_

A. 1st--Because gla.s.s is a _reflector_, and throws the candle-light _back into the room again_; and

2ndly--The pupil of the eye (which has become _contracted_ by the light of the room) is _too small_ to collect rays enough from the dark street, to enable us to _see into it_.

Q. _Why can't persons in the street SEE into a WELL-LIGHTED ROOM?_

A. Because the pupil of their eyes is _much dilated by the dark_, and cannot collect from the window sufficient rays to enable them to _see into the room_.

Q. _Why do we often see the FIRE REFLECTED in our parlour WINDOW in winter time?_

A. Because gla.s.s is a _good reflector_; and the rays of the fire (striking against the window-gla.s.s) _are reflected back into the room again_.

Q. _Why do we often see the shadow of our CANDLES in the window, while we are sitting in our parlour?_

A. Because the rays of the candle (striking against the gla.s.s) are _reflected back into the room_: and the _darker_ the night, the _clearer_ the reflection.

Q. _Why is this reflection more clear, if the external AIR be DARK?_

A. Because the reflection is not then _eclipsed_ by the brighter rays of the sun _striking on the other side of the window_.

Q. _Why is the SHADOW of an object (thrown on the wall) LARGER and larger, the CLOSER any object be held to the CANDLE?_

A. Because the rays of light _diverge_ (from the flame of a candle) _in straight lines_, like lines drawn from the centre of a circle.

[Ill.u.s.tration: Here the arrow A held close to the candle, will cast the shadow BF on the wall: while the same arrow held at C, would cast only the little shadow D E.]

Q. _When we enter a long AVENUE of TREES, WHY does the avenue seem to get NARROWER and narrower till it appears to MEET?_

A. Because the _further the trees are off_, the more _acute will be the angle_ that any two will make with our eye.

[Ill.u.s.tration: Here the width between the trees A and B will seem to be as great as the line AB: But the width between the trees C and D will seem to be no more than EF.]

Q. _In a long straight STREET, WHY do the houses seem to APPROACH NEARER and nearer as they are more DISTANT?_

A. Because the more _distant the houses_ are, the more _acute will be the angle_ which any two make with our eye.

Thus in the last figure--

If A and B were two houses at the top of the street, the street would seem to be as wide as the line A B:

And if C and D were two houses at the bottom of the street, the street at the bottom would seem to be no wider than E F.

Q. _In an AVENUE of TREES, WHY do they seem to be SMALLER as their distance increases?_

A. Because the _further the trees are off_, the more _acute will be the angle_ made by their perpendicular height with our eye.

[Ill.u.s.tration: Here the first tree A B will appear the height of the line A B; but the last tree C D will appear only as high as the line E F.]

Q. _In a long straight STREET, WHY do the houses seem to be SMALLER and smaller the FURTHER they are OFF?_

A. Because the _further any house is off_, the more _acute will be the angle_ made by its perpendicular height with our eye.

Thus in the last figure--

If A B be a house at the top of the street, its perpendicular height will be that of the line A B.

If C D be a house at the bottom of the street, its perpendicular height will appear to be that of E F.