Source: Charles Babbage, Passages from the Life of a Philosopher (London: Longman, Green, Longman, Roberts & Green, 1864), pp. 180-181
Text: One day Herschel, sitting with me after dinner, amusing himself by spinning a pear upon the table, suddenly asked whether I could show him the two sides of a shilling at the same moment.
I took out of my pocket a shilling, and holding it up before the looking-glass, pointed out my method. “No,” said my friend, “that won’t do;” then spinning my shilling upon the table, he pointed out his method of seeing both sides at once. The next day I mentioned the anecdote to the late Dr. Fitton, who a few days after brought me a beautiful illustration of the principle. It consisted of a round disc of card suspended between the two pieces of sewing-silk. These threads being held between the finger and thumb of each hand, were then made to turn quickly, when the disc of card, of course, revolved also.
Upon one side of this disc of card was painted a bird; upon the other side, an empty bird-cage. On turning the thread rapidly, the bird appeared to have got inside the cage. We soon made numerous applications, as a rat on one side and a trap upon the other, &c. It was shown to Captain Kater, Dr. Wollaston, and many of our friends, and was, after the lapse of a short time, forgotten.
Some months after, during dinner at the Royal Society Club, Sir Joseph Banks being in the chair, I heard Mr. Barrow, then Secretary to the Admiralty, talking very loudly about a wonderful invention of Dr. Paris, the object of which I could not quite understand. It was called the thaumatrope, and was said to be sold at the Royal Institution, in Albermarle-street. Suspecting that it had some connection with our unnamed toy, I went the next morning and purchased, for seven shillings and sixpence, a thaumatrope, which I afterwards sent down to Slough to the late Lady Herschel. It was precisely the thing which her son and Dr. Fitton had contributed to invent, which amused all their friends for a time and had then been forgotten.
Comments: Charles Babbage (1791-1871) was a British mathematician, philosopher and inventor. He is considered to be the father of computing for his work on his difference engines. The Thaumatrope was an optical toy popular throughout the nineteenth century. Two related images on either side of a card, when spun via a connecting thread, would appear to form a single a picture (such a bird and a cage coalescing to form a bird in a cage). The Thaumatrope appeared in 1825, supposedly the invention of British doctor John Ayrton Paris. Babbage’s anecdote suggests that William Henry Fitton (a geologist) was the inventor, inspired by the principle set out by the astrnomer John Herschel. Joseph Banks died in 1820, which ought to put this story in the 1810s, but it is assumed that Babbage’s memory was slightly at fault, with the mid-1820s being far more likely.
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