86 Chronicles of Science. [Jan., 



of the threads of the finder ; returning then to the large telescope 

 the observer brings one of the points of the micrometer behind one 

 of the principal lines of the star's spectrum. The star to be com- 

 pared with the first is then brought under the same thread of the 

 finder. If then the micrometer point coincides with a line of the 

 spectrum, this line and the line of the first star's spectrum are 

 evidently identical. 



One of the most remarkable results (assuming its correctness) 

 of Secchi's researches, is the observation that two stars — y Cassiopeia?, 

 and £ Lyra? — show bright lines. In y Cassiopeia?, for instance, 

 there are several bright lines, but one dominant line in the blue- 

 green, taking the place of a dark line — the well-known line F of 

 hydrogen — in other star-spectra. The spectra of these two stars 

 are compared by Secchi with the continuous spectrum crossed by 

 bright hues given by magnesium. 



The observation would seem to indicate that some stars owe 

 their light in part to the luminosity of their gaseous envelopes, and 

 notably to the presence of burning hydrogen. 



Before leaving the subject of spectrum-analysis, we must note 

 the investigation by M. Jansen, of Paris, of the formation of dark 

 lines when light passes through aqueous vapour. He has ascer- 

 tained that the intensity of certain hues seen in the solar spectrum 

 varies with the amount of moisture present in the atmosphere. By 

 transmitting the light of sixteen gas-burners through a tube filled 

 with steam he reproduced all these lines. Father Secchi appears 

 to have anticipated this discovery. 



M. Chacornac has published an interesting paper on Comets. 

 Space will not permit us to deal with the subject otherwise than 

 briefly. He compares together the atmospheres of the sun, of 

 planets, and of comets, under the several conditions of temperature 

 and attraction to which those atmospheres are subject. In the case 

 of planets it is possible that there should be an equilibrium between 

 the attractive force of the planet on the external layers of the 

 atmosphere, and the elastic forces of the layers below ; in such a 

 case the atmosphere will have a definite limit. But this clearly 

 cannot be the state of the atmospheres of comets near perihelion, 

 nor of the solar atmosphere. Beyond the bounds of the solar 

 attraction the forces of dilatation exhibit themselves as projective 

 forces acting outwards from the solar periphery. The rays of the 

 solar aureole, in total eclipses of the sun, indicate, by their configu- 

 ration, the expansive force of gases violently projected into plane- 

 tary space. To a similar expansive action, acting upon cometary 

 atmospheres, the formation of cometary aigrettes is attributed, 

 while the formation of comets' tails is ascribed to repulsion, pro- 

 duced by the expansive forces of the solar atmosphere. 



M. Leon Foucault has devised a new method of solar observa- 



