458 Prof. M. A. Corcm on the Distinction between 



Hence, if on the diameter OTD a point T' be taken such 

 that OT / = (l+i7)OT, and if through T' a parallel to AB is 

 drawn cutting OA, OB in A', B' respectively and DU, DV 

 in U', V respectively, we have U / V / = 2U / T / = (l-7 7 )(^+/) 7 

 and A'B / =2A / T / =(l+7 ? )(^-p / ); so that U'A'=Ei, and 

 WB'=W. 



Also, if with T' as centre and T'O as radius a circle be 

 drawn, it will pass through A', B', and will give the strains in 

 any directions, in the same way as the circle OAB gives the 

 stresses. For example, the stress on the plane OEE' has the 

 normal component UF, and the tangential component FE ; 

 while the normal strain is proportional to U / F / and the sliding 

 (g) to 2F / E / . 



The figure also shows the relation between E and the 

 coefficient (G) of elasticity of sliding ; for FE = Gg, and 

 2F / E' = E# ; therefore 



E : G = 2F / E':FE = 2(H-7 7 ). 



Coopers Hill, 



October 11, 1886. 



LIY. On the Distinction between Spectral Lines of Solar and 

 Terrestrial Origin. By Prof. M. A. Cornu*. 



[Plate VIII.] 



EKAUNHOFER, when he discovered the dark lines with 

 which the solar spectrum is crossed, gave them names 

 in order to facilitate description ; the principal lines were de- 

 signated by the letters A, B, ... H in such a way as to separate 

 approximately the seven principal colours of the spectrum. 

 The subsequent observations of Brewster, Dr. Gladstone, and 

 M. Janssen proved that, notwithstanding the symmetry of 

 denomination and the identity of appearance, these dark lines 

 belonged to two distinct classes. Indeed, the one preserves 

 always the same aspect, while the other becomes broader and 

 darker as the sun approaches the horizon. 



The first, most of which have been identified with the bright 

 lines due to metallic vapours (iron, magnesium, calcium, 

 nickel, &c), have been attributed, since the researches of 

 Prof. Kirchhoff, to the absorption produced by metallic sub- 

 stances in a state of vapour on the surface of the sun. The 

 other, in consequence of their intensity varying with the 

 thickness of the atmosphere traversed by the sun's rays, are 

 explained by the selective absorption due to the cold gases or 



* Communicated by the Physical Society : read June 12, 1886. 



