148 On Wave-lengths of Rays of High Refrangibility. [Apr. 19 



April 19, 1883. 



THE PRESIDENT in the Chair. 



The Presents received were laid on the table, and thanks ordered for 

 them. 



The following Paper was read : — 



I. " Measurements of the Wave-lengths of Rays of High Re- 

 frangibility in the Spectra of Elementary Substances." By 

 W. N. Hartley, F.R.S.E., &c., Professor of Chemistry, 

 Royal College of Science, Dublin, and W. E. Adeney, F.C.S., 

 Associate of the Royal College of Science. Communicated 

 by Professor G. G. Stokes, Sec. R.S. Received March 20, 

 1883. 



(Abstract.) 



The authors describe a method of taking photographs of diffraction 

 spectra produced by a small Rutherfurd speculum ruled with 17,460 

 lines to the inch. The lines in the spectra were accurately measured 

 by the aid of a microscope magnifying 25 diameters and a dividing 

 engine. 



The length of the spectra which were taken on three different plates 

 was 14 to 15 inches, and the measurements were accurate to the ao^oth 

 of an inch. From these measurements the wave-lengths of the lines 

 were calculated. The spectra include lines with wave-lengths 4674 

 and 2024. They were produced by electric sparks condensed by a 

 pane of glass coated with tin-foil. 



Of the electrodes used, one always consisted of cadmium, the other 

 of the metal or the solution of the metal, or other elementary 

 substance, the wave-lengths of the lines of which was to be determined ; 

 thus all the spectra were referable to the cadmium lines. Great 

 accuracy is attainable by this method, and lines which have appeared 

 identical or coincident in two different spectra, have thus been proved 

 to differ in refrangibility. 



All the spectra were compared with spectra obtained with the prism 

 spectroscope described by one of the authors in the " Scientific 

 Proceedings of the Royal Dublin Society," vol. iii, Part III, April, 

 1881. 



Great care was exercised in taking the photographs, lest any 

 irregularity in the surface of the plates should lead to inaccurate 

 measurements. Gelatine films on specially selected patent plate glass 



