By giving the page a revolving or rinsing motion the three circular figures printed on the next page appear to rotate. The best effect will be produced by laying the book down flat on the desk or table and revolving, first in one direction and then in the opposite direction, in such a way that any given point on the page will describe a circle of about 1/2 in. diameter. Fig. 1 then appears to rotate in the same direction as the revolution; Fig. 2 appears to revolve in the opposite direction, and Fig. 3 appears to revolve sometimes in the same direction and at other times in the opposite direction.
Move These Figures Rapidly With A Rinsing Motion
A curious effect can be produced with Fig. 1 by covering up Figs. 2 and 3 with a piece of plain paper and laying a coin or other small object on the paper. If the vision is then concentrated on the coin or other object while same is being revolved, Fig. 1 will be seen to rotate.
The engraving shows a perfectly straight boxwood rule laid over a number of turned brass rings of various sizes. Although the effect in the illustration is less pronounced than it was in reality, it will be noticed that the rule appears to be bent, but sighting along the rule from one end will show that it is perfectly straight.
Illustration: An Optical Illusion
The brass rings also appear distorted. The portions on one side of the rule do not appear to be a continuation of those on the other, but that they really are can be proved by sighting in the same manner as before. --Contributed by Draughtsman, Chicago.
When looking at the accompanying sketch you will say that the letters are alternately inclined to the right and left. They are not so and can be proved by measuring the distance of the top and bottom of any vertical strokes from the edge of the entire block. They will be found to be exactly the same distance. Or take any of the horizontal strokes of the four letters and see how far their extremities are from the top and bottom of the entire block. It will be found that a line joining the extremities of the strokes are strictly parallel to the top or bottom and that they are not on a slant at all. It is the slant of the numerous short lines that go to make up the letter as a whole that deceives the eye.
If a person observes fixedly for some time two balls hanging on the end of cords which are in rapid revolution, not rotation, about a vertical axis, the direction of revolution will seem to reverse. In some experiments two incandescent "pills" of platinum sponge, such as an used for lighting gas-burners, were hung in tiny aluminum bells from a mica vane wheel which was turned constantly and rapidly in one direction by hot air from a gas flame to keep the platinum in a glow. The inversion and reversion did not take place, as one might suppose, at the will of the observer, but was compulsory and followed regular rules. If the observer watches the rotating objects from the side, or from above or from below, the inversion takes place against his will; the condition being that the image on the retina shall be eccentric. It takes place also, however, with a change in the convergence of the optical axes, whether they are parallel to each other or more convergent. Also when the image on the retina is made less distinct by the use of a convex or concave lens, the revolution seems to reverse; further, in the case of a nearsighted person, when he removes his spectacles, inversion results every time that the image on the retina is not sharp. But even a change in the degree of indistinctness causes inversion.
Illusions Shown by Revolving Platinum Sponge "Pills" and Hat Pins
The cause of this optical illusion is the same where the wings of windmills are observed in the twilight as a silhouette. It is then not a question of which is the front or the back of the wheel, but whether one of the wings or the other comes towards the observer. The experiment is made more simple by taking a hat pin with a conspicuous head, holding it firmly in a horizontal position, and putting a cork on the point. Looking at it in semi-darkness, one seems to see sometimes the head of the pin, sometimes the point towards him, when he knows which direction is right. The inversion will be continued as soon as one observes fixedly a point at the side. Here it is a question of the perception of depth or distance; and this is the same in the case of the rotating balls; the direction of seeming revolution depends on which one of one considers to be the front one and which the rear one.
From the foregoing the following conclusion may be reached: When, in the case of a perception remitting two appearances, one fixedly observes one of these and then permits or causes change in the sharpness of the image on the retina, the other appearance asserts itself.
Can you tell which of these three figures is the tallest? Make a guess, and then verify its correctness by measurement.
After taking a look at the accompanying illustration you will be positive that the cords shown run in a spiral toward the center, yet it shows a series of perfect circles of cords placed one inside the other. You can test this for yourself in a moment with a pair of compasses, or, still more simply, by laying a point of a pencil on any part of the cord and following it round. Instead of approaching or receding from the center in a continuous line, as in the case of a spiral, you will find the pencil returning to the point from which it started.
Illustration: The Cord Is Not a Spiral
The accompanying sketch shows two optical illusions, the first having a perfect circle on the outside edge appears to be flattened at the points A, and the arcs of the circle, B, appear to be more rounding. In the second figure the circle appears to have an oval form with the distance from C to C greater than from D to D. A compass applied to the circles in either figures will show that they are perfectly round. --Contributed by Norman S. Brown, Chippewa Falls, Wis.
Illustration: The Two Illusions