6 THE POPULAR SCIENCE MONTHLY. 



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 ap- 

 pearances 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 re- 

 mains every way problematical. The extent of this vast solar appen- 

 dage 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 Colo- 

 rado. 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 spend- 

 ing 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 prob- 

 ably exists, but where M. Cornu finds our own atmosphere to inter- 

 pose 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 ex- 

 tremely important announcement that, reasoning from analogies fur- 

 nished by known compounds, he has been able to show that many ele- 

 ments 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- 



