The third feature of vision is the perception of color. Color is to light what pitch is to sound; that is, it depends on the vibration rate of the light waves. Light, as already explained, is one of the forms in which energy reaches the earth from the sun. Heat is another form. Both are portions of a great energy stream to which we give the name of _radiant energy_. This, as it comes from the sun, is made up of a mixture of vibrations having almost every imaginable rate, except that the slowest are many times faster than the highest pitched sound. At a certain rate, and one that for these vibrations ranks as slow, the energy is what we know as heat, and over a considerable range it continues to be called heat; more rapid vibrations, and they are so rapid that they have to be expressed in trillions per second, cause the effect on our retinas that we call light. The slowest that we can see give the sensation of red; the most rapid the sensation of violet; the other colors of the rainbow, which in order after red are orange, yellow, green and blue, are vibration rates between those that give red and those that give violet. It will be noticed that only six colors are given for the rainbow instead of seven. This is because there is not enough real difference between blue and indigo to justify making them separate colors. The distinction was made at the time when it was supposed that there was something specially wonderful about the number seven, which made it necessary that every important feature in nature should show that number. We now know of no reason why seven should have virtue over any other number. When all the vibration rates are mixed together, as they are in the sunlight, the sensation is white. There are other mixtures of colors that give white also, but they are not exactly equivalent to the white of sunlight, as is proven when one tries to match colors under artificial light. A great deal of labor has been devoted to the attempt to get an artificial light that shall be practically equivalent to sunlight, and only lately have good results been obtained. When no light enters the eye the sensation is of black, and it is worth while to note that so far as our sensations are concerned black is as much a color as white or any other, notwithstanding the fact that no light falls on the retina when the black sensation is being felt. Contrary to ordinary belief, blind persons who are blind because their eyes have been destroyed do not see black all the time; they simply have no sensations at all from the eyes. On the other hand, persons blind because of cataract do see black, because in them the retinas are still present, but no light falls on them.