Let a small perspective glass be made, that is wide enough at the end
where the object-glass is placed, to hold a table similar to the
following:—
┌───────────────────────┐
│ 1.131 10..132 19.133 │
│ 2.231 11..232 20.233 │
│ 3.331 12..332 21.333 │
│ │
│ 4.121 13..122 22.123 │
│ 5.221 14..222 23.223 │
│ 6.321 15..322 24.323 │
│ │
│ 7.111 16..112 25.113 │
│ 8.211 17..212 26.213 │
│ 9.311 18..312 27.313 │
└───────────────────────┘
Take a pack of cards that consists of 27 only, and giving them to a
person, desire him to fix on any one, then shuffle them and give the
pack to you. Place the twenty-seven cards in three heaps, by laying down
one alternately on each heap, but before you lay each card down, show it
to the person without seeing it yourself; and when the three heaps are
finished, ask him at what number, from 1 to 27, he will have his card
appear, and in which heap it then is. Then look at the heap through the
glass, and if the first of the three numbers which stands against that
number it is to appear at, say 1, put that heap at top; if the number be
2, put it in the middle; and if it be 3, put it at bottom. Then divide
the cards into three heaps, in the same manner, a second and a third
time, and his card will then be at the number he chose.
_Example._—Suppose he desire that his card shall be the 20th from the
top, and the first time of making the heaps he says it is in the third
heap; you then look at the table in the perspective, holding it at the
same time over that heap, and seeing that the first figure is 2, you
therefore put that heap in the middle of the pack. The second and third
times you in like manner put the heap in which he says it is, at the
bottom, the number each time being 3. Then looking at the pack with your
glass, as if to discover which the card was, you lay the cards down one
by one, and the twentieth card will be that he fixed on.