Page:Popular Science Monthly Volume 16.djvu/16

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an immense and hitherto almost unsuspected extension of the corona in the direction of the solar equator, such as to make it increasingly probable that the outer corona and the zodiacal light are different appearances with a common origin. The physical constitution of the inner corona seemed to be modified by the weakness or absence of a former constituent, and perhaps we may say that some additional knowledge was gained as to its telescopic structure and its absolute light, while the polariscopic evidence was contradictory.

In the light of our latest knowledge, what, then, is the corona? We do not know. We have literally had but about twenty minutes in the last twenty years to look at it, and from that brief study it remains every way problematical. The extent of this vast solar appendage is unknown, its constitution is unknown, its function is unknown, and it is still uncertain whether we can devise any means for its study which will free us from this dependence upon momentary glimpses. Our only hope, since the most powerful telescope is useless in our lower atmosphere, seems to be to transport our observatory to some mountain-height, like that of Etna or the elevated table-lands of Colorado. There, even, we can not be sure of seeing it without an eclipse; but there, if anywhere, ingenuity will be hopefully employed in an endeavor to remove the difficulties which bar the way. After spending some weeks this year myself upon Mount Etna, on which the new solar observatory is to be built, I can testify to the excellence of such a station; and yet, when we have sites equally good, I can not but regret that it should be left to others to first enter such a promising field.

Of recent spectro-photographic observation, I may mention the valuable work of M. Cornu, who, working at the other extremity of the spectrum from Captain Abney, has extended it beyond the violet to a wave-length of 2,900, far beyond which the solar spectrum probably exists, but where M. Cornu finds our own atmosphere to interpose an almost impassable barrier. The solar spectrum, therefore, is now known by photography through three times the extent of the visible portion, and this great gain on our former knowledge may be said to have been completed for us in the past year.

In last November and subsequently, Mr. Lockyer has made the extremely important announcement that, reasoning from analogies furnished by known compounds, he has been able to show that many elements are really compound bodies, which, incompletely dissociated at the highest temperatures we can command, furnish under the form of feeble lines the spectra of their components.

I do not enter here into discussion of points still in debate; but that which has arisen round this and the recent communications of Dr. Henry Draper, at any rate elicits the evidence of the immense labor now requisite in establishing new facts in our science, and the refinement of some of the adverse explanations suggested in contro-