224 Intelligence and Miscellaneous Articles. 



the use of a corrected lens, to avoid the absorption of flint-glass in 

 the achromatic combination. 



More or less absorption when glass is used is unavoidable ; and 

 this, with the variation of the focal plane of the lens for light of 

 different wave-lengths, constitutes the most serious defects of the 

 apparatus — defects which are avoided in the Eowland instrument. 

 However, compensating advantages are not wanting. The Eowland 

 apparatus integrates all impressions received at the slit. The 

 ordinary comparison-prism cannot be used with it, and the lower 

 orders of spectra are too narrow to admit of convenient division at 

 the camera ; in fact, the spectra begin at nothing at the slit, and 

 spread in a widening band as we move toward the higher orders. 



The new instrument possesses the advantages of the analyzing- 

 spectroscope. The spectrum can be made wide or narrow, or di- 

 vided at the slit. A preliminary trial has given the following 

 results : — A flat grating of 14,000 lines to the inch and a ruled 

 surface 2 x 1| inches was employed, with a lens of 37-feet focus. 

 The latter was placed close to the grating, and its spectrum ob- 

 served near the slit. In the second spectrum b 1 and b 2 were fully 

 an inch apart as projected on a screen, and D 1 and D 2 had a sepa- 

 ration of 13 to 14 millim. 



The excellence of the definition is shown by the fact that, with 

 an ordinary reading-glass of 6-inch focus, E was seen double, and 

 fourteen lines were counted between D 1 and D 2 . "With this 

 form of apparatus the amount of dispersion can be varied at 

 pleasure by simply altering the relative distances of slit and camera 

 from the grating, but in such a way that slit and camera shall 

 occupy con-jugate foci of the lens. — Silliman's American Journal, 

 July 1887. 



ON THE ELECTRIFICATION OF GASES BY IGNITED BODIES. 

 BY J. ELSTER AND H. GEITEL. 



The results of this investigation may be summed up as follows : — 



(1) The phenomenon that insulated conductors near an ignited 

 body are electrically charged holds also for gases, which have been 

 freed from dust by filtration through glycerine wadding. 



(2) This holds also even when the rarefaction is raised to the 

 extreme limits of a Crookes's tube. 



(3) The electrification is positive for red heat ; and all tempera- 

 tures beyond in all gases as yet examined, with the exception of 

 hydrogen, which at a high temperature has the opposite behaviour. 



(4) For air and carbonic acid the maximum of electrification is 

 at a yellow heat. 



(5) The layer of gas which surrounds an incandescent body 

 shows a different deportment as respects positive and negative 

 electricity. That electricity whose sign is opposite that developed 

 in the gas by ignition is most rapidly discharged. — Wiedemann's 

 Annalen, No. 5, 1887. 



