All these toys—and they may some day become more than toys—depend on a curious property of the human eye. The reader will remember that in the description of the human eye, as compared with the camera obscura as applied to photography, it was mentioned that the image was thrown from the front to the back, and in the one case was received on a naturally sensitive membrane, and in the other on a film rendered artificially sensitive by chemical means. This membrane is called the “retina,” because it not only receives the impression, but retains it for some little time after the object is removed. It has been calculated that the duration of the image is about the eighth part of a second.
Thus the eyelids are perpetually and unconsciously closing and opening with a rapid movement, popularly called “winking.” This movement is for the purpose of cleansing the eyeball, and, were it not for the image-retaining power of the retina, we should pass a considerable part of our time in absolute darkness. As it is, the impression of external objects on the retina lasts longer than the time occupied in winking, and, in consequence, we are not conscious that any interval of darkness has elapsed.
Again, when we have been looking steadfastly at an object, and then move our eyes, the image of that object is seen in the new focus; and it is worthy of notice that such object is always seen in its “complementary” colour. For example, if we have been looking at a scarlet spot, and suddenly move our eyes, we shall see a spot exactly similar in size and shape, but of green.
I well remember that when I was a boy I was reading with almost feverish anxiety the green handbill of a travelling circus, to which I hoped that I might be allowed to attend. Having finished it, I asked for some note-paper, for the purpose of putting my request in writing, but, to my astonishment, mixed, perhaps, with a little irritation, all the paper supplied to me was of a bright pink. For a time no arguments could convince me that the paper was really white, until by degrees the pink hue became paler and paler, and the paper assumed its normal whiteness.
The fact was, that the eye had become saturated with the green—i.e. the blue and yellow rays—and could see nothing but their complementary colour, which was pink.
A good example of this property may be found in a lighted stick, which, if rapidly whirled round, appears to form a continuous circle of fire. The reason of this is, that the impression made on the retina by the fiery point does not cease until the stick has again come round in its course.
Then there are those well-known chromatic tops, in which are inserted pieces of bent wire. When the top is spun these pieces of wire assume exactly the appearance of transparent jugs, vases, glasses, and similar articles. A very pretty illustration of this principle is given by a little machine, which is made to revolve rapidly by means of a multiplying wheel.
Upon its surface are fixed little pins, with polished globular steel heads, and, when the handle is turned, these heads form the most beautiful and intricate figures with exact accuracy.
Another toy, called the Thaumatrope, or Wonder-turner, is equally ingenious and beautiful, and is sufficiently simple to be made by any one with a slight knowledge of drawing. A disc of white cardboard is cut, and upon each side of it is portrayed some object. If the disc be caused to revolve rapidly, these two subjects will be seen at the same time, the image of each being held on the retina long enough to allow the other to take its place.
Some very beautiful combinations may be made by means of this instrument. For example, a horse may be on one side, and a man on the other, and, by spinning the disc, the man will be seen mounted on the horse. Then we may have a boat on one side, and a rower with his oars on the other. Similarly a mouse can be put into a trap, or a bird into a cage.