A Guide To The Scientific Knowledge Of Things Familiar Part 82

Q. _Why is the EYE PAINED by a SUDDEN light?_

A. Because the pupil of the eye is _burdened with rays_, before it has had time to contract.

Q. _Why does it give us PAIN, if a CANDLE be brought suddenly towards our BED at night time?_

A. In the dark _the pupils of the eyes dilate_ very much, in order to _admit more rays_. When a candle is brought before them, the enlarged pupil is _overladen with rays_, and feels pained.

Q. _Why CAN we BEAR the candle-light after a few moments?_

A, Because the pupil _contracts again_ almost instantly, and adjusts itself to the quant.i.ty of light which falls upon it.

Q. _Why can we SEE NOTHING, when we leave a WELL-LIGHTED room, and go into the DARK ROAD or street?_

A. Because the pupil (which _contracted_ in the bright room) does not _dilate instantaneously_; and the contracted pupil is not able to _collect rays enough_ (from the dark road or street) to enable us to see before us.

Q. _Why do we SEE BETTER, when we get USED to the dark?_

A. Because the pupil _dilates_ again, and is able to gather together more rays; in consequence of which, we see more distinctly.

Q. _If we look at the SUN for a few moments, why do all OTHER things appear DARK?_

A. Because the pupil of the eye (which was _very much contracted_ by looking at the sun) is _too small_ to collect sufficient rays from _other objects_, to enable us to distinguish their colours. (_See_ "accidental colours.")

Q. _If we watch a bright FIRE for a few moments, why does the ROOM seem DARK?_

A. Because the pupil of the eye (which was very much _contracted_ by looking at the fire) is _too small_ to collect sufficient rays from the objects around, to enable us to distinguish their colours.

Q. _Why can we see the PROPER COLOUR of every object again, after a few minutes?_

A. Because the pupil _dilates_ again, and accommodates itself to the light around.

Q. _Why can TIGERS, CATS, and OWLS see in the DARK?_

A. Because they have the power of _enlarging the pupil of their eyes_, so as to collect several scattered rays of light; in consequence of which, they can _see distinctly_ when it is not light enough for us to see _any thing at all_.

Q. _Why do CATS and OWLS SLEEP almost all DAY?_

A. As the pupil of their eyes is _very broad_, daylight _fatigues_ them; so they close their eyes for relief.

Q. _Why do CATS keep WINKING, when they sit before a FIRE?_

A. As the pupil of their eyes is _very broad_, the light of the fire _pains_ them; and they keep shutting their eyes to relieve the sensation of too much light.

Q. _Why do TIGERS, CATS, OWLS, &c. PROWL by NIGHT for prey?_

A. As these animals cannot see distinctly in _strong daylight_, they _sleep_ during the _day_: and as they can see clearly in the _dark_, they prowl then for prey.

Q. _Why do GLOW-WORMS glisten by NIGHT only?_

A. Because the light of day is _so much stronger_, that it _eclipses_ the feeble light of a glow-worm; in consequence of which, glow-worms are _invisible by day_.

Q. _Why can we not see the STARS in the DAY-TIME?_

A. Because the light of day is so powerful, that it _eclipses the feeble light of the stars_: in consequence of which, they are invisible by day.

Q. _Why can we see the STARS even at MID-DAY, from the bottom of a deep WELL?_

A. As the rays of the sun never come _directly over a well_, but the rays of the _stars_ do; therefore the light from those stars (in such a situation) is more clear than the light of the _sun_.

Q. _What is the USE of TWO EYES, since they present only one image of any object?_

A. The use of two eyes is to _increase the light_, or take in _more rays of light_ from the object looked at, in order that it may appear _more distinct_.

Q. _Why do we NOT see things DOUBLE, with TWO EYES?_

A. 1st--Because the _axis of both eyes is turned to one object_; and, therefore, the _same impression_ is made on the ret'ina of _each eye_.

2ndly--The nerves (which receive the impression) have _one point of union_, before they reach the brain.

Q. _Why do we SEE OURSELVES in a GLa.s.s?_

A. The rays of light from our face _strike against the surface of the gla.s.s_, and (instead of being absorbed) _are reflected_, or sent back again to our eye.

Q. _Why are the rays of light REFLECTED by a MIRROR?_

A. Because they cannot _pa.s.s through the impenetrable metal_ with which the back of the gla.s.s is covered; so they _rebound back_, just as a _marble_ would do if it struck against a wall.

Q. _When a marble is rolled towards a wall, what is that path THROUGH WHICH IT RUNS called?_