131.  How and Why Colors Change. Matching Colors. Most women prefer to shop in the morning and early afternoon when the sunlight illuminates shops and factories, and when gas and electricity do not throw their spell over colors.  Practically all people know that ribbons and ties, trimmings and dresses, frequently look different at night from what they do in the daytime.  It is not safe to match colors by artificial light; cloth which looks red by night may be almost purple by day.  Indeed, the color of an object depends upon the color of the light which falls upon it.  Strange sights are seen on the Fourth of July when variously colored fireworks are blazing.  The child with a white blouse appears first red, then blue, then green, according as his powders burn red, blue, or green.  The face of the child changes from its normal healthy hue to a brilliant red and then to ghastly shades.

Suppose, for example, that a white hat is held at the red end of the spectrum or in any red light.  The characteristics of white objects is their ability to reflect all the various rays that fall upon them.  Here, however, the only light which falls upon the white hat is red light, hence the only light which the hat has to reflect is red light and the hat consequently appears red.  Similarly, if a white hat is placed in a blue light, it will reflect all the light which falls upon it, namely, blue light, and will appear blue.  If a red hat is held in a red light, it is seen in its proper color.  If a red hat is held in a blue light, it appears black; it cannot reflect any of the blue light because that is all absorbed and there is no red light to reflect.

A child wearing a green frock on Independence Day seems at night to be wearing a black frock, if standing near powders burning with red, blue, or violet light.

132.  Pure, Simple Colors—­Things as they Seem.  To the eye white light appears a simple, single color.  It reveals its compound nature to us only when passed through a prism, when it shows itself to be compounded of an infinite number of colors which Sir Isaac Newton grouped in seven divisions:  violet, indigo, blue, green, yellow, orange, and red.

We naturally ask ourselves whether these colors which compose white light are themselves in turn compound?  To answer that question, let us very carefully insert a second prism in the path of the rays which issue from the first prism, carefully barring out the remaining six kinds of rays.  If the red light is compound, it will be broken up into its constituent parts and will form a typical spectrum of its own, just as white light did after its passage through a prism.  But the red rays pass through the second prism, are refracted, and bent from this course, and no new colors appear, no new spectrum is formed.  Evidently a ray of spectrum red is a simple color, not a compound color.

If a similar experiment is made with the remaining spectrum rays, the result is always the same:  the individual spectrum colors remain simple, pure colors. The individual spectrum colors are groups of simple, pure colors.