Page:Popular Science Monthly Volume 9.djvu/672

644 the eye. The term bright color is sometimes used in a somewhat analogous sense, but the ideas are so totally different that there is little risk of confusion.

The practical determination of the second constant is possible in a great many cases; it presents itself always in the shape of a rather troublesome photometric problem, capable of a more or less accurate solution. The relative brightness of the colors of the solar spectrum is one of the most interesting of these problems, as its solution would serve to give some idea of the relative brightness of the colors, which, taken together, constitute white light. Quite recently a set of measurements were made in different regions of the spectrum by Vierordt, who denoted the points measured by the fixed lines, as is usual in such studies. The following table will serve to give an idea of his results:

These measurements were made on a spectrum obtained by a glass prism, which, as has been mentioned in a previous chapter, contracts the red, orange, and yellow spaces unduly, and hence increases their illumination disproportionately. It is to be hoped that a corresponding set of measurements will soon be made on the normal spectrum, furnished by a ruled plate. If we should multiply the luminosity of the colors in either kind of spectrum by their extent or areas, we should obtain measures of the relative amounts of these several tints in white light.

By the simple method of rotating disks we can very roughly determine the second constant in the case of a colored surface, for example, of paper tinted with vermilion. A circular disk, about six inches in diameter, is cut from the paper, and placed on a rotation apparatus, as indicated in Fig. 3. On the same axis is fastened a double disk of black-and-white paper, so arranged that the proportions of black-and-white can be varied at will. When the whole is set in rapid rotation, the color of the vermilion paper will of course not