Though Fraunhofer had failed to grasp the true significance of the dark lines in the spectrum he was able to solve another highly important question—that of determining the wave-lengths to which these lines corresponded. From the wave-theory of light it may be readily understood that certain ether particles in the courses of different rays of light (e. g., those of equal amplitude) may receive a strengthening or retardation in their transverse vibrations according as they fall in with the same phase of vibration or out of it. Upon this principle of interference of light as developed by Young, Fraunhofer based his method for studying and measuring the lines of the spectrum. He made what he called a grating by ruling close together a number of parallel lines upon a glass plate. When light is thrown upon this series of equal and equidistant apertures a certain amount of the light will be diffracted to either side of the direct course. Among these diffracted rays as collected by a convex lens may be found several series of bright and dark bands which correspond to the points of augmentation and retardation, respectively, of the ether particles under the influence of light from certain apertures. By simple calculation the first bright band is known to be formed when the light rays from two adjacent apertures differ by exactly one wave-length in their respective courses to this band. A ready means, therefore, is given for measuring the wave-lengths of light rays. When white light is used a number of these bright bands will occur, with the light of shortest wave-length—the violet—nearest the central image and that of the longest wavelength—the red—farthest removed. In other words, we have a spectrum, but one so constructed that a direct means is given for determining the wave-lengths of the various lines it may present. The complete map of the wave-lengths of the lines in the visible solar spectrum was published in 1868 by Angstrom. The wave-lengths were expressed in ten millionths of a millimeter. Since that time they have served as a standard in all similar investigations under the name of the Angström Unit (A.U.). One millimicron (the millionth of a millimeter μμ,) is equal to 10 A.U. The visible spectrum extends from light of about 7,600 A.U. in the red to that of about 3,900 A.U. in the violet. A more satisfactory method of expressing the results of observations in the spectrum is to use the number of waves of any particular ray of light which occur in one centimeter in vacuo, or what is called the oscillation frequency (O.F.). This is the reciprocal of the wave-lengths when reduced to vacuum values. As the reduction makes but little difference in the final value, it is usually customary to make the correction by adding one or two A.U. to the observed wave-lengths. Curves constructed from oscillation frequencies approach more nearly a straight line, and thus are easier to draw.
A few of the best known lines may be given in order to show the relation in values: