Interference Methods in Spectroscopy 83 SUMMARY 1. The spectrum of the light emitted by incandescent gases is not continuous, but is made up of a number of bright lines whose position in the spectrum is very definite, and which are characteristic of the elements which produce them. 2. These “lines” are not such in a mathematical sense, but have an appreciable width and a varying distribution of light, and in some cases are highly complex. 3. This variation in distribution is, however, restricted to such narrow limits that in most cases it is impossible to investigate it by the best spectroscopes; but by the method of visibility curves the lines may be resolved into their elements. 4. An important auxiliary for the interpretation of the visibility curves is the harmonic analyzer—an instrument which sums up any number of simple harmonic motions, and which also analyzes any complex motion into its simple harmonic elements. | LECTURE V LIGHT WAVES AS STANDARDS OP LEXGTH In the last lecture it was shown that in many cases the interference fringes could be observed with a very large difference in path—a difference amounting to over 500,000 waves. It may be inferred from this that the wave length, during the transmission of 500,000 or more waves, has remained constant to this degree of accuracy; that is, the waves must be alike to within one part in 500,000. The idea at once suggests itself to use this invariable wave length as a standard of length. The proposition to make use of a light wave for this purpose is, I believe, due to Dr. Gould, who mentioned it some twenty-five years ago. The method proposed by him was to measure the angle of diffraction for some particular radiation — sodium light, for example—with a diffraction grating. If we suppose Fig. GO to represent, on an enormously magnified scale, the trace of such a grating, then the light for a particular wave length — say one of the sodium lines— which passes through one of the openings in a certain direction, as AB, is retarded, over that which passes through the next adjacent opening, by a constant difference of path; and therefore in the direction AB all the waves, even those which pass through the last of a very large number of such apertures, are in exactly the same phase. There will be then, if we are observing in a spectrum of the first order, as many waves in this distance AB as there are apertures in the distance AC. A diffraction grating is made by ruling upon a glass or a metal surface a great number of very fine lines by a ruling diamond, the number being recorded by the ruling-machine 84 |