C. GENERAL PHYSICS. 153 



the theses defended by him at the close of his inaugural dis- 

 sertation, we note that the Ketteler formula for dispersion is 

 maintained by liim as the most correct that is known ; as 

 also that the appearance of the aurora is dependent upon 

 local climatic conditions. Blockh Inaugural Dlsseriatioii, 

 1873. 



SPECTRUM dispersio:n^ of gases. 



Lorenz communicates to the Royal Academy of Copen- 

 hagen an experimental and theoretical investigation into the 

 chromatic dispersion of gases. His observations have been 

 made by means of a small spectroscope, and were in part 

 directed toward the constituents of the atmosphere, for 

 which he gives the coefficients of refraction for each of the 

 eight principal lines of the spectrum, and the necessary 

 changes due to the moisture and temperature of the atmos- 

 phere. Pure oxygen, nitrogen, hydrogen, the vapor of water, 

 alcohol, ether, chloroform, iodaethyl, carbon-sulphide, and am- 

 monia are successively investigated by him in different de- 

 grees of density and temperature. Mem. Kongel. Danske 

 Yidensk. Selsk.^ Co2oenhagen,^.^ ^QQ. 



IMPEOYEMEXT IX SPECTROSCOPES. 



Mr. Madan gives a simple method by which we may cor- 

 rect the curvature of the spectrum lines, a defect inherent in 

 all spectroscopes as at present made. This curvature is due to 

 the fact that the rays from different parts of the spectrum fall 

 on the prism under different vertical angles. Mr. Grubb has 

 lately proposed to correct the curvature by a method which 

 had been already employed by Mr. Madan for more than a 

 year past, and which consists in making the slit itself curved 

 instead of straight, the curvature being so arranged as to 

 neutralize the distorting effect of the prism. Mr. Madan's 

 experience shows that the curved slits give perfectly satis- 

 factory results, and that they may be easily adapted to any 

 spectroscope ; so that by having several such slits in succes- 

 sion, we may secure straight lines in the field of view for 

 varying powers of dispersion. He finds that slits whose 

 edges are curved to a radius of 21 centimeters, sensibly cor- 

 rect the distortion of a single carbon bisulphide prism, the 



a 2 



