INTENSITY RELATIONS IN THE SPECTRUM OF HELIUM. 



145 



The experimental measures of the heights of the various lines in each case are given 

 in the following table, together with the density gradients of the wedge per millimetre 

 at each wave-length, determined from the graph of density gradient in the manner 

 already described. The table contains all the experimental data from which the 

 ensuing conclusions are drawn. As these conclusions are of a very general nature, it 

 was thought necessary to give the measurements relating to one set of photographs 

 in complete detail. 



The photographs are described by Roman numerals. 



TABLE I. 



The magnification of the various photographs was not completely identical in all 

 cases. On the original plate, the distance between the lines \X6678, 3888, was 

 44'95 mm. If this distance be measured on any individual photograph, its 

 magnification m is deduced by simple division. On photographs I.-V. inclusive, we 

 found m = 3'181, and on photographs VI.-VIIL, m = 3'159. 



Some of the Helium bands appear on the intermediate photographs, though absent 

 very close to the cathode, and again at some distance from it. They are, in fact, only 

 shown on the photographs IV.-VII. , according to the details set forth in Table II. The 

 wave-lengths are only rough values serving to identify the individual bands, and the 

 values of d x are obtained as before from the calibration curve of the wedge. We 

 merely record in the table some of the more conspicuous examples of these bands, as 

 an illustration of their behaviour. We. do not propose to discuss them, for they are 

 in reality band heads consisting of a large number of nearly overlapping lines, and 



VOL. ccxx. A. Y 



