PHYSICS. 4g7 



H. Becquerel has examined the emission-spectra of metallic vapors 

 in the infra-red region, making use of a suitable phosphorescent sub- 

 stance, previously rendered luminous by insolation, as the screen for 

 receiviug these spectra. Under the influence of the infra-red rays a 

 temporary excitatioH preceding extinction is developed, and the lines 

 otberwise invisible appear brilliant. Certain of these phosphorescent 

 powders, notably calcium sulphide, were so sensitive as to allow the wave- 

 lengths of the more brilliant lines of potassium, sodium, and cadmium 

 to be determined at once by means of a Eutherfnrd grating. In other 

 cases the spectra were obtained by means of a carbon disulphide prism, 

 the wave-lengths beiug determined by interpolation. The wavelengths 

 of 4 lines of i)Otassium, 1 of sodium, 5 of strontium, 2 of calcium, 4 of mag- 

 nesium, 2 of aluminum, 2 of zinc, 1 of cadmium, 5 of lead, 2 of silver, 

 2 of tin, and 1 each of bismuth and thallium are given. (C. B., xcix, 

 374, August, 1884 ; Phil. Mag., October, 1884, V, xviii, 386 ; Am. J. Sci., 

 December, 1884, III, xxviii, 457.) 



Becquerel has applied the same method to the investigation of the 

 infra-red region of the solar spectrum. A beam of light reflected from 

 a diffraction grating was concentrated by a lens and traversed a car- 

 bon disulphide prism whose sides were normal to the slit and the lines 

 of the grating, and formed on the phosphorescent surface a series of 

 oblique spectra, in which the radiations with spectra of difierent orders 

 were juxtaposed and not superposed. The slit was so narrow that the 

 principal lines of the luminous spectra could be seen, so that by com- 

 paring the lines and bands in the infra-red of the first spectrum with 

 the known lines of the second and third their wave-lengths could be 

 obtained. The author claims that this method allows of a further ex- 

 ploration than the photographic one, and is not exceeded by the bolom- 

 eter or thermopile. Indeed, if the phosphorescent agent is sensitive, 

 the details can be made out far better than with the bolometer. The 

 wave-lengths measured are given in the paper. {G. B., xcix, 417, Sep- 

 tember, 1884; Phil. 31ag., November, 1884, V, xviii, 465; Am. J. ScL, 

 III, XXVIII, 391, 459.) 



Soret and Sarasin, members of a commission appointed by the Natural 

 History Society of Geneva to study the properties of the water of their 

 lake, have observed a distinct absorption band in its spectrum, which 

 is visible when the light passes through a layer of the water only 2™ 

 in thickness. This band is a little less refrangible than D, beiug 

 located in the orange, about one-fifth the distance between D and C, 

 corresponding to a wave-length of about 600. This band was observed 

 in distilled water even when carefully purified by distillation with per- 

 manganate in a vessel of platinum. (0. B., March, 1884, xoviii, 624.'i 



Soiet has published a later paper, in which the question of the color of 

 water is discussed with especial reference to the color of the Lake of 

 Geneva. {J. Phys., October, 1884, II, iii, 427.) 



Abney and Festing have shown that, when light traverses a thick- 



