Q. _How are FISHES able to ASCEND to the SURFACE of water?_ A. Fishes have an _air-bladder_ near their abdomen: when this bladder is _filled with air_, the fish increases in size; and (being lighter) ascends through the water to its surface. Q. _How are fishes able to DIVE in a minute to the BOTTOM of a stream?_ A. They _expel the air_ from their air-bladder; in consequence of which, their _size is diminished_, and they sink instantly. CHAPTER XXVII. LIGHT. Q. _What is LIGHT?_ A. Rapid undulations of a fluid called ether, striking on the optic nerve of the eye. (_See p. 46._) The _heat_ of fire or of the sun sets the atoms of _matter_ in motion; and these atoms, striking against the fluid _ether_, cause it to undulate. Q. _How FAST does LIGHT TRAVEL?_ A. Light travels so fast, that it would go eight times round the earth, while a person counts "ONE." Q. _Does ALL light travel equally fast?_ A. Yes; the light of the sun, or the light of a candle, or the light from houses, trees, and fields. Q. _Where does the LIGHT of HOUSES, TREES, and FIELDS come from?_ A. The light of the _sun_ (or of some lamp or candle) is _reflected from their surfaces_. Q. _Why are SOME surfaces BRILLIANT like glass and steel, and OTHERS DULL like lead?_ A. Those surfaces which _reflect the most light_, are the most _brilliant_; and those which _absorb_ light are _dull_. Q. _What is meant by REFLECTING LIGHT?_ A. Throwing the rays of light _back again_, from the surface on which they light. Q. _What is meant by ABSORBING LIGHT?_ A. Letting the rays of light _sink below the surface_ which they touch, so as not to be seen. Q. _Why can a THOUSAND persons SEE the SAME OBJECT at the same time?_ A. Because it throws off from its surface _an infinite number of rays in all directions_; and one person sees _one_ portion of these rays, and another person _another_. 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 quantity 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?_