L. Bell — Ultra-violet Spectrum of Cadmium. 427 



ible spectrum. As the overlapping spectra of different orders 

 are focussed in precisely the same plane, and the great focal 

 length of the grating ensures a perfect, focus in photographs 

 over a foot in length, the method of coincidences can be used 

 with the greatest exactness throughout the ultra-violet. 



In using this apparatus the method employed was in detail 

 as follows : 



The first step was to determine with the greatest possible 

 accuracy, the wave-lengths of the principal visible lines in the 

 spectrum of cadmium. As the source of light, poles were made 

 from cadmium which had been subjected to fractional distilla- 

 tion in vacuo, and the spark taken between them from a good 

 sized induction coil furnished with a quart Leyden jar and 

 worked by twelve Bunsen cells. The poles were separated 

 somewhat over a millimeter and placed as close as possible to 

 the slit. The slit also received the image of the sun from a 

 heliostat and quartz lens and was closed until either the solar 

 or the cadmium spectrum could be seen with equal sharpness 

 of definition. Then the cadmium line to be measured was 

 put near the center of field of the eyepiece micrometer and 

 micrometer readings were taken on solar lines of which the 

 wave-lengths were very accurately determined. When the 

 place of the cadmium line was reached the sunlight was cut 

 off' for a moment and a reading was taken on the cadmium. 

 Then the light was readmitted and the measurements on the 

 solar lines continued. 



This process was repeated several times and since the utmost 

 care was taken that the light from the poles and from the sun 

 should fall upon the grating in exactly the same manner, the 

 micrometer readings gave the wave-length of the cadmium line 

 with a very high degree of accuracy. The error of a wave- 

 length thus determined can in no case be as great as one part 

 in a hundred thousand, and usually it is much less. Inde- 

 pendent measurements made on different days do not differ by 

 more than this amount. 



The solar lin'es on which the exactness of the work depends 

 are determined to one part in half a million so far as relative 

 wave-length is concerned. The cadmium lines thus measured 

 were seven in number, and had wave-lengths, 6438*77, 5338-50, 

 5379-22, 5086-09, 4800-15, 4678-39, 441419, being Mascart's 

 first seven lines. 



Next in order came the determination of the extreme ultra- 

 violet rays. Cadmium fortunately has a very prominent group 

 of lines near wave-length 2300. Therefore a photograph of the 

 last three lines mentioned above, taken in the first order, would 

 include this ultra-violet group in the second order. And since 

 both orders were in focus together, the wave-lengths could be 



