A. Because bluish green is the "ACCIDENTAL COLOUR" of red: and if we fix our eye upon _any colour whatsoever_, when we turn aside, we see every object tinged with _its accidental colour_. Q. _If we wear BLUE GLASSES, (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 glasses_, we shall see its "_accidental colour_," when we lay our glasses 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 glass for spectacles the BEST for wear?_ A. Because _white_ is the accidental colour of _black_; and if we wear _black glasses_, 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 glasses are _white_, and black being its "accidental colour," every thing appears in a _black shade_, when we lay our glasses 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 compasses, 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.