636 ■ TRANSACTIONS OF SiiCTlOK A. 



A Email vacuum tube, made from capillary tubing and fitted with platinUui 

 electrodes, Avas filled to a pressure of about 1 mm. of mercury, with radium 

 emanation purified with Kutherford's apparatus and method.' A quartz con- 

 densing lens was employed, and the iron arc used to give a comparison spectrum. 

 Three photographs were obtained, two being of the end-on discharge seen througli 

 a quartz window. 



About thirty- five of the more intense lines previously obtained by liuther- 

 ford and Royds ^ with a glass prism can be seen on the photographs, together 

 with four additional lines further in the ultra-violet. The spectrum presents the 

 same characteristics as those previously obtained, and the measurements are in 

 good agreement. The error in the wave-lengtiis of the grating pliotograph^t is 

 not more than about O'l A. I'. 



G. I'hotoyraphs of Arc Spectra of Metals under I'ressitre. 

 By Dr. W. G. Dufkield. 



7. Secondary Effects in the Echelon Spectroscope. By H. Stanspield, B.Sc. 



Further experiments made with the echelon spectroscope described at the 

 Jieicester meeting of the Association have shown that some of the variations in 

 the spectra have their origin in the instrument. For example, a dark line that is 

 (li'ten observed in the bright central band of the given mercury line spectrum, and 

 has been supposed to be an absorption line, can be moved across from one side of 

 Ihe band to the other by a slight rotation of the echelon about a vertical axis. 



These effects are found to be due to the superposition on the primary spectrum 

 of .secondary light that lias been twice reflected in the echelon. 



If a bright line spectrum is examined with the echelon in the ordinary 

 position, the secondary effects are not conspicuous, as the secondary lines are 

 parallel to the spectrum lines and can only be formed on a fairly broad band such 

 as the central band of the green mercury line ; but on rotating the echelon about 

 a vertical axis the secondary lines will be seen crossing the broad bands in the 

 spectrum as they move in the same direction as the spectrum lines and move 

 much faster. 



When the echelon is raised at one end the secondary lines become con- 

 spicuous ; they are more inclined than the spectrum lines, in the same direction, 

 and give them a screw-like appearance. 



With the echelon in this position it is not difficult by means of screens to 

 obtain the secondary spectrum fi-ee from primary. It is also possible to obtain 

 the primary spectrum so free from secondary light that the spectrum lines lose 

 their screw-like appearance. 



The secondary action of the echelon in forming a spectrum by internal 

 reflection is similar to that of a Fabry and Pei'ot spectroscope. This action alone 

 would throw the secondary light into a ring spectrum, but as it emerges from 

 the step faces of the echelon the light also undergoes the ordinary echelon treat- 

 ment and so it is confined to the points of intersection of lines representing the 

 same wave length in the ring spectrum and the echelon spectrum. The secondary 

 lines observed in the spectrum resulting from the superposition of the primary 

 and secondary spectra are the loci of these points of intersection. 



The distance apart of the secondary lines, their motion when the echelon is 

 rotated, and their inclination when one end of the echelon is raised have been 

 calculated in accordance with this theory, and the results agree well with the 

 observations. 



'J'he use of the secondary point spectrum extends the resolving power of the 

 instrument and gives a method of verifying the measurements made in the usual 

 way, 



' Rutherford, ' Experiments with Radium Emanation,' Phil. May., August 1908. 

 - Rutherford and Royds, ' Spectrum of Radium Emanations,' ibid. 



