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 which in passing through the prism are refracted or bent from their direct course, and some are bent more than others, the red least of all and the violet most. It is supposed that light is propagated by waves or undulations in an extremely rare substance termed ether which is supposed to occupy all space and transparent bodies. These waves are thought to be similar

to sound waves in the air or the ripples on the smooth surface of a pond when a pebble is thrown into it. Because so many of the phenomena of light can be satisfactorily explained by this theory, it has been very generally adopted by the scientists. The amount that rays of light are refracted from a straight line in passing through a prism is in proportion to the number of waves or undulations per second, and in inverse proportion to the length of the waves. The reel waves are refracted the least and are the longest, while the violet rays are refracted the most and are the shortest.

Whether this theory of the spectrum formation is absolutely correct or not, the fact is established that the colors found in a prismatic solar spectrum are always the same under the same conditions and the order of their arrangement is never changed. By means of the quality of spectrum colors called the wave length, a given color can always be located in the spectrum, and hence if a spectrum color is selected as a standard it can always be determined by its recorded wave length.