n8 POPULAR SCIENCE MONTHLY 



Though Fraunhofer had failed to grasp the true significance of the 

 dark lines in the spectrum he was able to solve another highly impor- 

 tant 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 augmenta- 

 tion and retardation, respectively, of the ether particles under the in- 

 fluence 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 wave- 

 length — the red — farthest removed. In other words, we have a 

 spectrum, but one so constructed that a direct means is given for deter- 

 mining 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 



o 



Angstrom Unit (A.U.). One millimicron (the millionth of a milli- 

 meter up,) 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: 



