820 J. D. Dana on the supposed Legs of a Trilohite. 



Furthermore, those other lines of iron, which are often seen in 

 the chromosphere spectrum, are nearly all of about the same 

 order of prominence as 1474. The more conspicuous iron lines 

 seldom if ever appear reversed, while in the case of other sub- 

 stances their strongest spectral lines are always the first to turn 

 hright. 



If then we admit a sufficient repulsive force, it seems still 

 possible to suppose that the leucosphere may consist of iron in 

 the state of vapor and fog ; and the well known wide diffusion 

 of this metal in meteoric matter makes it comprehensible how 

 its lines should occur in the spectrum of our own terrestrial 

 aurora, and in any other places where they may be found. 



Possibly future researches in the laboratory may throw clearer 

 light upon the subject. 



It is hardly necessary to add that our own terrestrial atmos- 

 phere, when char, appears to me to play only a very subordi- 

 nate part in the phenomenon. Some influence it must, of course, 

 have ; but remembering how much the inner portion of the 

 coronal ring exceeds in brilliance the outer, it would seem 

 that the illumination of the lunar disc must give us an exag- 

 gerated measure of the true atmospheric effect. This illumina- 

 tion makes the edge of the moon only enough brighter than the 

 center to give it the appearance of a"' globe, but of almost inky 

 blackness. 



With the subjective element, the case is very different. In 

 untrained observers especially, it may be so influential that 

 two intelligent persons standing side by side will describe and 

 even sketch upon paper appearances most grotesquely different 

 from each other and from the truth. 



Even skilled observers are greatly affected by the strangeness 

 and pecuhar nature of the phenomenon, and the excitement ol 



Art. XLVIL— Cm the supposed Leqs of the TrUohite, Asaphn^ 

 I>latycephahis ; by James D. Dana. 



At the request of Mr. E. Billings of Montreal, I have re^ 

 cently examined the srecimen of Asaphus ])Iati/cephalus i-'^^' 

 longing to the Canadian Geological Museum, which has beeii 

 supposed to show remains of legs. Mr. Billings, while he i|J^ 

 suspected the organs to be legs so far as to publish on the si 

 ject,* has done s"o with reserve, saying, in his paper, "thatth 

 first and all-important point to be decided, is whether or no 

 ..:.?„£?.«l.S-i ^- IH P.- ^'^^ 1870. with a plate giving a full-sized -jj"' 



