Now for a word as to Professor Michelson’s suggestion that an unvarying unit of measurement may be found in a certain light-wave, as observed in the interferometer. Everybody knows that each chemical element burns with colors of its own. When we see red fire bursting from a rocket we know that strontium is ablaze; when the tint is green it tells us that copper is on fire, as when a trolley-wheel jumps from its electric wire. When these sources of light are looked at through an accurate prism of glass in a spectroscope they form characteristic spectra, and these spectra in their peculiarities of color reveal what elements are aflame. In most cases the rays from an element form a highly complicated series; to this rule cadmium, a metal resembling zinc, is an exception. It emits a red, a green, and a blue ray; the wave-lengths of these rays Professor Michelson proposes as a basis of reference for the metallic standards of length adopted by the nations of Europe and America. He says: “We have in the interferometer a means of comparing the fundamental standard of length with a natural unit--the length of a light-wave--with about the same order of accuracy as is at present possible in the comparison of two metre-bars, that is, to one part in twenty millions. The unit depends on the properties of the vibrating atoms of the radiating substance, and of the luminiferous ether, and is probably one of the least changeable qualities in the material universe. If therefore the metre and all its copies were destroyed, they could be replaced by new ones, which would not differ among themselves. While such a simultaneous disaster is practically impossible, it is by no means sure that notwithstanding the elaborate precautions that have been taken to ensure permanency, there may not be slow molecular changes going on in all the standards, changes which it would be impossible to detect except by some such method as that here presented.” Thus, by dint of mechanical refinements such as the world never saw before, some of the smallest units revealed to the eye become the basis of all measurement whatever, reaching at last those cosmical diameters across which light itself is the sole messenger. In the early days of spectroscopy many doubters said, What good is all this? Since then a full reply has been rendered to their question and, at this unexpected point, the spectroscopic examination of an unimportant metal may afford a measuring unit of ideal stability. Cases like this suggest the query, Is any knowledge whatever quite worthless? CHAPTER XVII MEASUREMENT--_Continued_ Weight, Time, Heat, Light, Electricity measured with new precision . . . Exact measurement means interchangeable designs, and points the way to utmost economies . . . The Bureau of Standards at Washington . . . Measurement in expert planning and reform. [Illustration: Ancient Egyptian balance.] The Balance in Measurement.