WOKK OF HENRI BECQUEREL BROCA. 775 



another compound. It would be a true method of spectrum analysis 

 which could be employed in researches relative to the rare earths. 



After having shown upon many crystals the fruit fulness of his 

 method, Becquerel closed his research in this line by, as usual, 

 reuniting- into a theory the facts observed. The intensity of a 

 ray after having traversed a unit thickness of a crystal must be 

 i= (a COS.- a + ^ C0S.2 ^ -\- c cos.- y)^ for a given wave length, the ray 

 making the angles a, (3, and y, with the principal absorption axes, the 

 intensity observed along the three axes being a, h, and c. But there 

 are cases where two absorption bands are superposed in the same part 

 of the spectrum having different principal directions of absorption. 

 Then the photometer measures must be represented by the product of 

 two expressions of the same form. Thus we may have an asymmetric 

 curve for the intensity as a function of the angles a, /?, y. This takes 

 place in epidote, and Becquerel was able to explain the apparently 

 paradoxical results of Ramsay in making photometric measures for 

 various orientations in the plane, g^, of epidote. 



We see in this series of researches the same qualities of mind 

 present in the earlier one ; extremely delicate experiments directed by 

 theoretical ideas and the final embodiment in a theory rendering 

 numerical account of the observed facts. The problem brought to 

 this point had no further interest for Becquerel ; he left to others the 

 patient work of appljdng his methods. He himself started on a 

 new path, one already laid out for him, which, in a sense, was an 

 heritage. Edmond Becquerel had already made discoveries upon it 

 of the first order. In phosphorescence Henri Becquerel found a sub- 

 ject well adapted to his trend of mind and where his radical ideas 

 could bear full fruit. We find again the double line of work, the 

 theoretical and the practical, side by side. He showed the first in 

 establishing a new method for the spectroscopic analysis of flames, 

 the other in discovering two distinct laws : First, the law connecting 

 the radiation engendering phosphorescence and that emitted from 

 the phosphorescing body; second, the law connecting the diminution 

 of the emitted energy with the time. This series of experiments is 

 also of the first rank, for the study of the constitution of matter for 

 phosphorescence is certainly intimately connected with molecular 

 resonance. Phosphorescence results from selective absorption, but 

 the nature of the radiation is such that a purely temperature con- 

 nection between the phosphorescent emission and the nature of the 

 body's absorption is an insufficient explanation of the emission, which 

 is itself selective. These phenomena seem related to those of the 

 selective emission of incandescent vapors, and so it was natural to 

 look for laws analogous to those governing the luminous emission of 

 gases and vapors. It is true that the molecules are less free in solid 

 88292— SM 1908 50 



