359. =Thaumatrope.=--Each impression made upon the optic nerve by light lasts about the eighth part of a second. No distinct impressions can be made, therefore, upon the retina unless they succeed each other with less rapidity than this. If, for example, in the revolution of a wheel, eight or more spokes pass by one point in a second, they can not be seen as distinct spokes, but will be mingled together, producing one continuous impression. So, too, if a light revolve so as to describe a circle in an eighth part of a second it will appear to the eye as one unbroken circle of light. It is this continuous impression on the retina that makes small objects, as the cars pass swiftly along, appear to run in long lines along with us. The fact thus developed is made use of in the contrivance of a toy called the thaumatrope. A picture is made on each side of a circular card, and whirling the card around very rapidly by means of two strings fastened to it, the two pictures are made to mingle together as one. Thus in Fig. 249 are represented the two sides of such a card, on the one side there being the picture of a dog, and on the other that of a monkey. When made to revolve rapidly the monkey will be seen sitting on the back of the dog. [Illustration: Fig. 250.] 360. =Light Compound.=--I have thus far spoken of light as if it were a simple thing. But it is compound. Every ray of white light has in it seven different colors. That this is so we can prove by taking a beam of light by itself and dissecting it, as we may say, or separating it into its seven parts. I will show you how this can be done. Let D E, Fig. 250, a beam of the sun's light, pass through a small opening in a shutter into a dark room. The rays will pursue a straight course, and if a screen be placed at F they will make a spot of white light. But if a glass prism, A B C, be held in the position represented the rays will be refracted, and when received upon the screen M N the light will be separated into seven colors in the order which is given. The figure thus produced is called the solar spectrum. Observe why it is that the colors are separated. It is because they are refracted unequally. If they were equally refracted the light upon the screen would be white, as before it was refracted. The violet rays are most refracted, the indigo next, the blue next, etc., and the red are the least refracted of all. 361. =Proportion of the Colors in Light.=--The colors in light are not equal in amount. If we divide the spectrum into 360 equal parts the proportion in the colors will be as follows: red, 45; orange, 27; yellow, 40; green, 60; blue, 60; indigo, 48; violet, 80. Some suppose that there are really but three simple colors, red, yellow, and blue, the other colors being produced by a combination of these. Thus red and yellow will together form orange, and yellow and blue will form green. [Illustration: Fig. 251.]