You can make another top, after a similar fashion. Cut your postcard as before, making one half of it black. Now, out of the white side, cut a segment with an angle of 45 degrees, leaving a little piece near the center as shown in [Fig. 13]. This piece you have left has nothing to do with the effect, but is simply to make the top revolve better.
Take an ordinary book, of which the printing is presumably black, and revolve the top upon the pin at the rate of about five turns a second (a sharp twitch with the finger will do this perfectly well). If you look at the printing now you will find the letters are colored red, as though the book had been printed in red ink!
In both these experiments the alternation of black and white has not only confused the eye, but has deceived it into seeing colors which do not really exist.
So we have shown very conclusively that the old proverb, “all is not gold that glitters,” can be applied to even the plainest of black and white; and, as the poet remarks, “things are not what they seem.”
Therefore, when people wish to impress you with the evidence of their own eyes and clinch an argument by stating that they saw such and such a thing and cannot be wrong, show some of these optical illusions and demand a better proof of what they affirm, very courteously assuring the dogmatist that the best of eyes are liable to make mistakes.
CHAPTER LXIII
CIPHERS AND CRYPTOGRAMS
Successful Systems of Secret Writing
From the earliest times secret writing has been considered no less an art than a necessity. Innumerable have been the systems invented and the means employed to insure the secrecy of messages and instructions. Yet in the passage of time by far the greater number of these methods of cipher has become obsolete and practically useless, failing in most cases to comply with the three great necessities which Bacon declared to be indispensable to all ciphers and cryptograms: (1) Easy of reading and writing; (2) difficult of solution; and (3) void of suspicion.
Ciphers may be generally divided into two branches—code ciphers and letter ciphers. The first of these terms refers to systems so arranged that one group of characters represents several words or sentences, whilst the other term designates those cryptograms where each letter in every word has its corresponding symbol.