344 Scattering and Regular Reflexion of Light. 



regarding the exact structure of the line from these results, 

 though Malinowski was able to make some interesting calcu- 

 lations as to the probable widths of the component lines. 



We have made a preliminary investigation with the inter- 

 ferometer of the structure of the 2536 line of the water- 

 cooled mercury arc and the resonance radiation of the vapour 

 at room temperature. The results are not as satisfactory as 

 we hoped for, owing to the low reflecting power of the 

 metals in this region of the spectrum. Mirrors of silicon 

 were tried, but if thick enough to exhibit the high reflecting 

 power found by Hurlbut, their absorption was too great. 

 With mirrors of cathodically deposited cadmium we obtained 

 fair results, however. 



The interference fringes disappeared completely when the 

 Fabry and Perot plates were separated by a distance of 6 mm., 

 and re-appeared with a separation of 12 mm., both for the 

 water-cooled arc and the resonance radiation. 



If we consider that the resolving power of the plates is 

 no greater than that of a Michelson interferometer, our 

 inference from this result would be that we were dealing 

 with a doublet with a separation of about 0*03 A. 



As a matter of fact, the resolving power was somewhat 

 greater, as the fringes obtained with small path difference 

 were distinctly narrower than those of a system formed by 

 two interfering beams. The finite widths of the components 

 of the assumed doublet may, however, be sufficient to account 

 for their disappearance with a 6 mm. path difference^ and 

 their diminished visibility at 12 mm.; so that, for the present 

 at least, we are justified in considering the line a doublet. 



It is obvious that a further study of the line with a higher 

 resolving power will be necessary before any positive state- 

 ment can be made. This we intend to do in the autumn 

 with a plane grating of 90,000 lines in the lOth-order spec- 

 trum, which should give us a resolving power of about *0028. 



On looking over the plates made in this region with the 

 very powerful quartz spectrograph of Professor Weiss with a 

 resolving power equivalent to that of 13 large 60° quartz 

 prisms, it seems as if the study of the line with this instru- 

 ment would scarcely be worth while, for the resolving power 

 is only about '033. On this account it will be necessary to 

 use a large grating unless we succeed in getting something 

 better in the way of a metal film for the interferometer. 



In this investigation we have been greatly aided by a 

 substantial grant from the Solvay Institute of Physics of 

 Brussels. 



