On the application of interference methods to the study to the study of the origin of certain spectrum lines

Abstract

In recent years, great progress has been made in the study of the nature of spectra and of spectral series, but it may be said that very little is yet known as to the nature of the luminous particles from which different spectrum lines originate. It is generally supposed that band spectra are in some way due to molecules, whilst series spectra are usually associated with the atom. Enhanced lines were for many years supposed to be due to proto-elements, or simplified forms of the chemical atom, a view which has recently to some extent fallen into discredit. The evidence for all these hypotheses is of a circumstantial nature, and very little definite evidence as to the nature of the luminous particles is available. In their recent important researches, Buisson and Fabry have opened up a new method of attacking the problem. The method adopted by these investigators consists in measuring the limiting order of interference at which fringes can be observed for different radiations. The limiting order of interference depends on the widths of the spectrum lines, from which certain deductions may be made with regard to the temperature of the source of the radiations, and the masses of the particles which are concerned in their production. The theory of the method has recently been discussed by Lord Rayleigh and by Schönrock. The chief circumstance which need be considered as determining the widths of spectrum lines produced in gases at low pressures is the Doppler effect due to the motion of the luminous particles in the line of sight. The researches of Michelson have shown that at higher pressures a broadening of the lines occurs, this broadening being attributed to disturbances caused by collisions between the luminous particles, but that at pressures below one thousandth of an atmosphere this cause of broadening may be considered negligible.