344 ANNUAL OF SCIENTIFIC DISCOVERY. 



Spectrum of the Nebula in Argo. — From the examination by the 

 spectroscope by the expedition under Lieut. John Herschel to the 

 southern hemisphere, we are informed that the great nebuki in 

 Argo exhibits a spectrum of bright lines, so that, like its splendid 

 r.orthern rival, the great Orion nebula, this object is gaseous. 



Great Nebula in Orion. — According to Father Secchi's observa- 

 tions, this nebula is much better seen in moonlight than on dark 

 nights. This surprising statement he makes as the consequence 

 of that optical principle, that the difference of two lights is more 

 easily appreciated when they are weak than when both are strong. 

 He finds that the spectrum of hydrogen, as Mr. Huggins antici- 

 pated, may be made, by suflBciently diminishing the light, to pre- 

 sent the middle line only, which is that visible in the nebula. 



Stellar Spectra. — Father Secchi, in a recent communication to 

 the French Academy, draws attention to a fourth type of stellar 

 spectra, which had hitherto escaped him from the small size of the 

 stars, and the imperfection of instruments. The essential charac- 

 ter of this type is the appearance of three luminous bands, separated 

 by obscure intervals. The brightest band is in the green, gener- 

 ally well-defined and greatly dilated. Another band, much feebler, 

 presents itself in the blue, often visible with difiiculty ; the third 

 band is found in the yellow, and widens toward the red, only it is 

 subdivided into many others. All these have the characteristic 

 that their light goes on increasing from the side of the violet, 

 where they stop suddenly. On the contrar}^ toward the red they 

 present a gradual diminution of color to perfect black. There is, 

 therefore, a complete opposition between this type and the third ; 

 for in the latter the columns are not only double in an equal space, 

 but they show the maximum of light on the side of the red, and 

 the minimum toward the violet. Tiie two spectra, consequent- 

 l}', are not a modification of a single type, but are evidently due 

 to completel}" different substances. What are the substances which 

 produce these phenomena cannot be exactly defined, but they 

 present a remarkable analogy to the reversed spectrum of car- 

 bon. 



In general, red stars have zoned spectra; the smaller have 

 often a continuous one ; this continuity may be only apparent, and 

 the stars may belong to the red tyj)e without zones, as in Arctu- 

 rus among the larger stars. The rays of hydrogen coincide pre- 

 cisely with the four black rays of aLyrae ; even the most refrangible 

 ray of the violet is found in its place in the spectrum of hydrogen ; 

 another secondary line of hydrogen appears in the SjDectrum of 

 this star, and there can be no doubt that this substance forms its 

 absorbent atmosphere, and that the star has no appreciable proper 

 motion. The coincidence of the ra3's of hydrogen with those of 

 the light of the sun, reflected by the moon, is as precise as for 

 a Lyrae. 



Specti-um of Aixturus. — According to M. Secchi, the spectrum 

 of Arcturus presents the same fine metallic rays as a Orion, and 

 a Scorpion (Antan-s) . The appearances which render, at first sight, 

 these spectra so different, depend on wide bands of absorption, which 

 arc absent in Arcturus, well marked in a Orion, and still more de- 



