The variation in the brightness of periodic stars has by some been supposed to be due to an opaque body periodically obscuring the light. Should that body be surrounded by an atmosphere like our planet’s, its presence would be revealed by the absence or presence of additional lines of absorption in the spectrum of the star. Now three lines determined in the spectrum of Betelgeux were no longer found when the star arrived at its maximum of brightness, indicating it may be the presence of an atmosphere round the opaque body. With regard to our own planets, Jupiter has lines in his spectrum which indicate the existence of an absorptive atmosphere; one band indicates the presence of vapours similar to those existing in our atmosphere, another band has no counterpart among the lines of absorption of the earth’s atmosphere, and tells of some gas which it does not contain. In the feeble spectrum of Saturn there are lines similar to those in the spectrum of Jupiter. These lines are less strongly marked in the ansæ of the rings, and show that the absorptive power of the atmosphere about the rings is less than that of the atmosphere which surrounds the ball. M. Jansen has found lines denoting aqueous vapour in the atmospheres of both Jupiter and Saturn. Some very remarkable lines have been seen in the more refrangible part of the spectrum of Mars supposed to be connected with his red colour. Though the spectrum of Venus is brilliant, and the dark lines distinct, no additional lines indicate the existence of an atmosphere differing from our own. The phenomena resulting from an examination of the nebulæ are most wonderful; their light is very feeble, even that of the brightest. ‘The total light of the whole nebula in Orion, the largest and brightest of them, makes so small an impression on the naked eye, that you may look twenty times at its place and not perceive any nebulous light at all.’[23] Besides, the brightness of a surface cannot be increased by a telescope, however good. Notwithstanding difficulties which seem to be almost insurmountable, Mr. Huggins in England, and Padre Secchi at Rome, have been, and still are, engaged in these researches.