6 FLUORESCENCE OF THE URANYL SALTS. 



rays from entering the eye on the other, and thus by means of a prism 

 held to the eye was able to observe the spectra of both fluorescence and 

 absorption with surprising accuracy. 



Paragraph 148 of his paper describes his observations on uranyl 

 nitrate. In the following quotation of that paragraph certain pas- 

 sages forecast in an extraordinary manner some of the conclusions 

 reached in subsequent chapters of the present monograph: 



"The sun's light was reflected horizontally by a mirror and condensed by 

 passing through a large lens. It was then transmitted through a vessel with 

 parallel sides containing a moderately strong ammoniacal solution of a salt 

 of copper. The strength of the solution and the length of the path of the 

 light within it were such as to allow of the transmission of a little green besides 

 the blue and violet. 



"A crystal of nitrate of uranium was then attached to a narrow slit and 

 placed in the blue beam which had been transmitted through the solution, 

 the crystal being turned toward the incident light. The light coming from 

 the crystal through the slit was then viewed from behind and analyzed by a 

 prism. A most remarkable spectrum was then exhibited, consisting from end 

 to end of nothing but bands arranged at regular intervals. The interval 

 between consecutive bands appeared to increase gradually from the red to the 

 violet, just as is the case with bands of interference. Although this interval 

 appeared to alter continuously from one end of the spectrum to the other, 

 the entire system of bands was made up of two distinct systems, different in 

 appearance and very different in nature. The less refrangible part of the 

 spectrum, where, only for the crystal, there would have been nothing but 

 darkness, was filled with narrow bright bands due to the light that had changed 

 its refrangibility. The more refrangible part of the spectrum was occupied 

 by the system of bands of absorption. The interval between the most refrang- 

 ible light band and the least refrangible dark band of absorption appeared to 

 be a very little greater than one band interval, so that had there been one 

 more band of either kind the least refrangible absorption band would have 

 been situated immediately above the most refrangible bright band. With 

 strong light I think I have seen an additional band of this nature." 



Becquerel, in the course of his work on phosphorescence, notes the 

 fact that most of the compounds of uranium show a strong emission 

 of light when examined with the phosphoroscope. He determined 

 the duration as three to four thousandths of a second; and to test his 

 empirical formulae made measurements of the rate of decay which, as 

 will be seen, are in good agreement with the results described in 

 Chapter IV of the present treatise. With a prism of carbon bisulphide 

 he observed 8 bright bands in the spectrum of the phosphorescent light 

 of uranyl nitrate; he determined the approximate range in the violet 

 and ultra-violet of the exciting rays; noted that the bands in the 

 spectra of various uranyl salts, such as the chloride, fluoride, and 

 uranyl potassium sulphate, occupy different places. He also esti- 

 mated their relative displacements when compared with the bands of 

 the nitrate. By comparing the spectrum of the nitrate during excita- 



