WORK OF HENKI BECQUEREL BROCA. 773 



reproduced mechanically if a transparent body could be turned at a 

 speed of millions of turns per second. 



In this complete series of researches, conducted by Becquerel 

 between his twenty-second and thirty-fifth years, we see the develop- 

 ment of all the qualities of a true investigator — the directing theo- 

 retical ideas, the consummate experimental skill, the discussion of all 

 the interesting results, those which concern the cosmical conditions of 

 our existence together with their practical applications, and, finally, 

 the theoretical coordination. This work, completed at 35, ranked 

 Henri Becquerel as a master and gave cause for the glorious career 

 which opened before him. 



At the time of these recondite researches he was preparing others, 

 all tending toward the study of the constitution of matter. They 

 were to treat of the absorption of light and of phosphorescence. 

 These led him to the discovery of the new rays. 



Let us examine next his work on crystalline absorption. AMiile 

 examining the absorption spectra of various crystals he noted that 

 the bands disappeared for certain orientations of the luminous vibra- 

 tion. Pushing further the study of this phenomenon, he saw that in 

 all double-refracting crystals having absorption bands similar phe- 

 nomena occur, and that the absorption in general is symmetrical about 

 three principal axes ; the more complicated the crystalline structure, 

 the more complicated the law of absorption. There is, however, one 

 general law binding them all. The bands observed through the same 

 crystal have invariable positions in the spectrum; their intensity 

 alone varies. 



In uniaxial crystals the phenomenon is symmetrical about one axis. 

 The absorption spectrum, in whatever direction observed, is formed 

 bv the superposition of two series of bands, one corresponding to the 

 vibration normal to the axis, the other to those parallel to it. For 

 every ordinary ray — that is, for every ray normal to the axis — the 

 absorption spectrum is the same for the same length of path. For 

 every extraordinary^ ray of which the vibration is orientated in the 

 plane of the ray and the axis the absorj^tion spectrum is as if the two 

 components of the ray normal and parallel to the axis individually 

 suffered absorption and then united upon emerging. 



In biaxial crystals one law is common to both orthorhombic and 

 clinorhombic crystals. Each band has three axes of rectangular sym- 

 metry. When the Fresnel vibration coincides with one of them, the 

 absorption band is at a maximum ; with another, it has a mean value ; 

 and with the third it is generally invisible. In orthorhombic crystals 

 the three directions of absorption coincide Avith the directions of sym- 

 metry of the crystal. In clinorhombic crystals the phenomena are 

 more complex and interesting. The axis of symmetry is always a 



