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XIII. The Grating Spectrum of Radium Emanation. By 

 T. Royds, M.Sc, 1851 Exhibition Scholar, Manchester 

 University *. 



IN order to measure with greater accuracy the wave-lengths 

 of the lines in the spectrum of radium emanation, it is 

 desirable to photograph the spectrum with an instrument of 

 the highest power practicable. The fact that when a dis- 

 charge is passed through a vacuum-tube containing pure 

 radium emanation, the emanation rapidly disappears into the 

 glass walls of the tube, sets a limit to the dispersion which 

 can be employed. Using, however, a concave grating 

 suitable for faint spectra, having a radius of only one metre, 

 it has been possible to photograph the more intense lines in 

 the spectrum of radium emanation, and also, incidentally, to 

 extend the spectrum farther into the ultraviolet than has 

 previously been photographed. 



This concave grating, which has been fully described 

 previously t, has a width of 3*5 inches, and is ruled with 

 15,000 lines to the inch. Its radius of curvature is 1 metre, 

 and the dispersion in the first order spectrum amounts to 

 16'8 A.U. per mm. The grating, the slit, and the camera 

 are mounted in a light-tight wooden case and the photographs 

 are taken on a photographic film w T hich is bent to the 

 requisite curvature by the shape of the film-carrier. 



A small vacuum-tube of about 50 c.rnms. capacity made out 

 of capillary tubing and fitted with platinum electrodes, was 

 filled to a pressure of about 0'1 mm. with radium emanation 

 purified by the method and apparatus of Prof. Rutherford J. 

 A quartz condensing lens was employed and the iron arc used to 

 give a comparison spectrum. The comparison spectrum was 

 obtained alongside on the same film by moving a suitably perfo- 

 rated screen in front of the slit between the two exposures §. 



* Communicated by Prof. E. Rutherford, F.R.S. 



t de Watteville, Phil. Trans. A 204. p. 139. 



X Rutherford, " Experiments with Radium Emanation," Phil. Mag. 

 Aug. 1908. 



§ This method waa employed for two reasons : (1) to avoid the risk 

 of displacing the film -carrier into which the comparison shutters 

 commonly used with the grating were slid in front of the film ; and (2) 

 in the case of the end-on discharge when the image of the tube was a 

 faint dot on the slit, to avoid the possibility of the height of the spectrum 

 being such that the spectrum was entirely cut out by the shutter — an 

 adjustment which it is difficult to carry out in the dark, and, moreover, 

 every moment of exposure is valuable. 



This method of using different portions of the slit for the two 

 exposures is open to objection if the slit is not exactly parallel to the 

 rulings of the grating owing to the astigmatism ; the adjustment was, 

 however, carefully made by Kayser's method, and, in any case within 

 reason, the error would be small. 



