A Guide To The Scientific Knowledge Of Things Familiar Part 90

Q. _If we wear BLUE GLa.s.sES, (when we take them off,) every thing appears tinged with ORANGE,--WHY is this?_

A. Because _orange_ is the "_accidental colour_" of blue: and if we look through _blue gla.s.ses_, we shall see its "_accidental colour_," when we lay our gla.s.ses aside.

Q. _If we look at the SUN for a few moments, every thing seems tinged with a VIOLET colour,--WHY is this?_

A. Because _violet_ is the "accidental colour" of _yellow_ light; and as the sun is _yellow_, we shall see its "accidental colour" _blue_, when we turn from gazing at it.

Q. _Does not the DARK SHADOW (which seems to hang over every thing after we turn from looking at the sun) arise from our eyes being DAZZLED?_

A. Partly so: the pupil of the eye is _very much contracted_ by the brilliant light of the sun, and does not adjust itself immediately to the feebler light of terrestrial objects; but, independent of this, the "ACCIDENTAL COLOUR" of the sun being _dark violet_, would tend to throw a shadow upon all things. (_See p. 366._)

Q. _Why is BLACK gla.s.s for spectacles the BEST for wear?_

A. Because _white_ is the accidental colour of _black_; and if we wear _black gla.s.ses_, every thing will appear _in white light_, when we take them off.

Q. _Why does every thing seem shadowed with a BLACK MIST, when we take off our common SPECTACLES?_

A. Because the gla.s.ses are _white_, and black being its "accidental colour," every thing appears in a _black shade_, when we lay our gla.s.ses down.

The accidental colour of red is bluish green.

" " " of orange " blue.

" " " of violet " yellow.

" " " of black " white.

And the converse of this is true:--

The accidental colour of bluish green is red.

" " " of blue " orange.

" " " of yellow " violet.

" " " of white " black.

(The law of an accidental colour is this--The accidental colour is always half the spectrum. Thus, if we take half the length of the spectrum by a pair of compa.s.ses, and fix one leg in any colour, the other leg will hit upon its accidental colour.)

N. B. The spectrum means the seven colours--Red, orange, yellow, green, blue, indigo, and violet, divided into seven equal bands, and placed side by side in the order just mentioned.

CHAPTER XXVIII.

SOUND.

Q. _What is SOUND?_

A. The vibration of some sonorous substance produces motion in the air called SOUND WAVES, which strike upon the _drum of the ear_, and give the sensation of sound.

Q. _What are MUSICAL SOUNDS?_

A. Regular and uniform successions of vibrations, which are always pleasing to the ear.

Q. _How FAST does SOUND TRAVEL?_

A. About 13 miles in a minute, or 1142 feet in a second of time.

Q. _How fast does LIGHT TRAVEL?_

A. Light would go 8 times round the whole earth, while sound is going its 13 miles.

Q. _Why are SOME things SONOROUS, and others NOT?_

A. The sonorous quality of any substance depends upon its _hardness and elasticity_.

Q. _Why are COPPER and IRON SONOROUS, and not LEAD?_

A. Copper and iron are _hard and elastic_; but as lead is neither hard nor yet elastic, it is _not sonorous_.

Q. _Of what is BELL-METAL made?_

A. Of _copper and tin_ in the following proportions:--In every 5 pounds of bell-metal, there should be 1 lb. of tin, and 4 lbs. of copper.

Q. _Why is this mixture of tin and copper used for BELL-METAL?_

A. Because it is much _harder_ and more _elastic_ than either of the pure metals.

Q. _Why is the SOUND of a bell STOPPED by TOUCHING the bell with our finger?_

A. The weight of the finger _stops the vibrations_ of the bell; and as soon as the bell _ceases to vibrate_, it ceases to make sound-waves in the air.

Q. _Why does a SPLIT BELL make a HOa.r.s.e disagreeable sound?_

A. The _split_ of the bell causes a _double vibration_; and as the sound-waves _clash and jar_, they impede each other's motion, and produce discordant sounds.