402 Dr. F. P. Kerschbaum : Interference 



distances d + an from the interference apparatus (where d 

 is the distance between the interference apparatus and one 

 of the resonating systems, n an integer, and a an arbitrary 

 constant). Only then could the trains arrive at the inter- 

 ference apparatus with a constant difference in phase, or in 

 the same phase if a=-2. But any such arrangement of 

 molecules is incompatible with the conception of a random 

 distribution of the molecules in a gas, independent of any 

 proper motion of these molecules ; 



(2) or if the Hg-molecules with their random distribution 

 in space take up the energy of the incident waves at an 

 individual and very special rate, so that the radiating systems 

 begin to discharge the trains of oscillation in a most com- 

 plicated and peculiar succession of moments only. 



Then again we could imagine that the different units- 

 arrive at the interference apparatus with a constant lack in 

 phase. But we have every reason to assume that this special 

 process is beyond the limit of any probability. 



So we can be sure that resonating Hg- vapour will behave 

 just like an independent source of light. 



The Optical Arrangement. 



The method of producing interference-fringes for our 

 purposes must fulfil the condition that the light does not hit 

 matter before it has passed the interference system. This- 

 condition is fulfilled in Young's arrangement : Light from a 

 line source passes through a double slit and afterwards falls 

 upon a screen. In the case of wave radiation we get — as is 

 known sufficiently well — on the screen a system of fringes. 

 The pattern can be explained by the conception that the 

 double slit acts as two independent sources of radiation. 

 Each slit produces its own diffraction pattern on the screen. 

 Both diffraction patterns overlap and, in doing so, give rise 

 to the production of interference-fringes, because the light 

 passing through the double slit is in a fixed phase relation. 

 The spectra of different order, as produced by the diffraction, 

 are crossed with lines of greatest intensity separated by 

 lines of smallest intensity in places where the path difference 



of the rays from the two slits is m\ and *^ 9 — \ respec- 

 tively (m being an integer). w 



If, however, light units with no definite phase relation 

 between them pass through the double slit, the formation of 

 a diffraction or interference pattern cannot take place. On 

 the screen we would have only to expect two lines of light 

 in places where the straight lines drawn from the source o£ 



