﻿and Absorption by Resonating Gas Molecules. 707 



after its passage through the cell ? A rigorous calculation 

 involves some difficulties, but we can make an approximate 

 estimate in the following way. In the experiment as it was 

 tried, the thickness of the cell was 10 mm., and the intensity 

 of the primary beam was therefore reduced to one quarter of 

 its original value, or in other words 75 per cent, of the 

 energy was scattered, and 25 per cent, reached the plate as 

 direct rays and recorded itself on a circular area 4 mm. in 

 diameter. It is evident that the intensity of the scattered 

 resonance radiation from the first layer of molecules en- 

 countered by the primary beam will be most intense, but its 

 intensity will be reduced when it reaches the plate, by its 

 passage through the gas, in the same proportion as is the in- 

 tensity of the primary ray, which excites the last layer of 

 molecules to a much feebler luminosity. The intensity of all 

 of the layers as seen by the photographic plate would thus ap- 

 pear to be equal. It is now evident that the 75 per cent, of the 

 energy diverted from the primary beam, since it is given out 

 in ail directions, is distributed over a complete sphere, and we 

 shall probably go not very far wrong if we assume it all as 

 coming from the first layer, which will make our sphere of 

 radius equal to 10 mm. (the thickness of the cell), for in 

 the actual experiment the photographic plate was placed in 

 contact with the anterior window, and not at a distance as 

 shown in the figure. The area of the sphere is 1256 mm. 

 and the area illuminated by the primary beam 12 mm., the 

 ratio of the areas being as 1 is to 105. We have then a 

 radiation of intensity 3 distributed over area 105 (scattered), 

 and radiation of intensity 1 concentrated on area 1. The 

 direct image ought, therefore, to be about 35 times as bright 

 as the corona. I made a rough measurement of this ratio 

 and found it to be in the neighbourhood of 1 : 40, which 

 agrees as well as we could expect, considering the way in 

 which the calculation was made. The calculation was made 

 on the assumption that no true absorption occurred, and the 

 fact that there is some agreement between the calculated and 

 the observed ratio indicates that we are dealing with pure 

 scattering when the mercury vapour is in a high vacuum. 

 The destruction of the corona by the admission of air is the 

 direct consequence of the introduction of the factor of true 

 absorption. I am of the opinion, however,- that the ratio of 

 the intensity of the secondary radiation to that of the primarv, 

 furnishes us with a better method of determining the ratio, and 

 as I have already mentioned, the value obtained leads me to 

 think that in a high vacuum there is no true absorption. A 

 rigorous calculation of the ratio under specified conditions 



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