Feb. 5j 1885] 



NA TURE 



3'5 



avoidable impurity only, are usually the most incisive and brilliant 

 part of the display. 



In a paper, however, on " Micrometrical Measures of Gaseous 

 Spectra," kindly printed, but not yet published, for me by the 

 Royal Society, Edinburgh, there is a full description of a case 

 which will be found to supply exactly the practical details that 

 may strengthen M. Bredichin's views. 



After having had tubes, and tubes, and tubes again of CH 

 gas, of various varieties of CH and at various pressures made 

 by different makers, and having found their CH spectrum (under 

 the electrical incandescence which M. Bredichin also assumes for 

 his comets) always more or less imperfect and more and more 

 haunted, often overpowered, by the brilliant lines of pure hydrogen, 

 I followed out the indications of least failure by eventually having 

 made a so-called vacuum, but really four inches of mercury 

 pressure tube of olefiant gas. It was constructed for me, with 

 very peculiar attention to, and precautions for, purity, by 

 Mr. Charles F. Casella, of 147, Holborn, and attained absolute 

 success at last, for no trace of any impurity whatever could I 

 discover in it from one end of the spectrum to the other. 



Not only so, too, bat the spectrum which it did show was the 

 most brilliant and perfect one of CH that I have ever heard of. 

 Every one knows the five diversely coloured bands of CH, four 

 of them first well described by Prof. Swan in 1856 ; each band 

 beginning towards the red with strong lines and bright haze, 

 which fades off towards the violet side into black, vacant space 

 long before the next band begins. And many persons know 

 that with greater spectroscopic power that haze is capable of 

 being resolved into a system of smaller lines, and far closer, or 

 linelets, but still coming to an end considerably short of where 

 the next band begins. 



But on this occasion, with the extra heavy olefiant gas-tube, 

 strong induction sparks, and a spectroscope having 24° dispersion 

 from A to H of solar spectrum, a telescopic magnifying power 

 of 14, a very narrow slit and excellent definition of prisms, 

 the linelets, usually so difficult to identify, were as sharp and 

 clear as luminous needles and continued, in a series of regularly- 

 increasing spaces apart, the whole distance from one CH band 

 to the next. This completeness was distinctly proved, first with 

 the Orange band and its needle-like scale of linelets (after all 

 its strong lines had been left behind), extending up so as to 

 touch, as it were, the brilliant beginning of the Citron band. 

 Then came its bright lines and closely-packed linelets continually 

 widening in distance apart, but losing nothing in sharpness and 

 definition until the Green band was reached. With the Green 

 band was its leader (the so-called "Green Giant of Carbo- 

 hydrogen ") burning like a pillar of electric fire ; then its close 

 linelets ; then its second line and linelets rather wider apart ; 

 its third line and linelets still wider ; and onwards linelets wider 

 yet, but preserving admirable regularity of series all the way, 

 all the long way, without missing, or slurring, one step, the 

 whole distance right up to the beginning of the Blue band. 



i el over one part of that lengthy road something extraneous 

 did appear; vaguely at first, or as a mere faint ghost of a 

 barely perceptible roll of gray-coloured cirrostratus cloud ! 

 Could it be subjective only ? possibly the reflection from a 

 fatigued part of the retina of the observer's eye. Not that, 

 for the linelets of CH were still brilliantly sharp, thin, and 

 narrow everywhere. What then? I, who had condemned scores 

 of vacuum tubes of all the gases for being filled with II lines, 

 had never seen anything like that floating, filmy cloud before ! 



I '.ut thought is quicker than sight. A suspicion of the truth 

 flashed in a moment upon me ; and on turning to the Red end 

 of the Spectrum, there, over the known place of Hydrogen's 

 Red line, was another faint broad region of barely visible 

 luminous haze, but reddish, in place of, like the other, a blue- 

 gray. Even, too, as I watched them, from that moment on 

 through an hour, first turning to one and then to the other, 

 tin t- haze-clouds narrowed and narrowed towards their central 

 verticals, whilst the sharp little linelets of the CH pari passu 

 became paler and paler, until at last they only remained visible 

 in the neighbourhood of the bigger lines and strong beginnings 

 of their respective bands. And by that time the once faint 

 clouds, the red and the gray, had become transformed into two 

 piercingly bright lines of Hydrogen light, the representatives of 

 Solar C and Solar F ; while the carbon of the CH, which the 

 II had been eliminated from by the action of the electric spark, 

 was deposited on the inside of the tube as a brown glaze. 



This, then, is the case of independent observation which I 

 beg to hand over to M. Bredichin for discussion, believing it to 

 illustrate that 



(1) In the condition most suitable for showing the CH, or 

 ordinary cometic, spectrum, — no H should appear. 



1 -I It a little free H should be introduced into a full atmosphere 

 of C 1 1, the characteristic lines of H are at first so extra broadened 

 — though seen under the same circumstances that those ofCH 

 are ultra narrow and defined in — as to be weakened thereby 

 below visibility, unless, indeed, the CH spectrum at the time be 

 almost infinitely brighter than it has ever yet been found in any 

 Comet. 



(3) 'Hie longer the incandescing electric influence is at work, 

 the greater is the evolution of pure H on one side, and deposition 

 of solid C on the other, out of CH gas. Whence we may 

 possess for the future an indicator for the comparative age of 

 Cornets ; or, at least, may pretty certainly conclude Halley's 

 Comet to be older than that of 1SS1, if bright, narrow lines of- 

 pure H, with or without CH bands accompanying, shall ;be 

 visible in the spectrum of the former at its next return : that, 

 in itself a consummation long most earnestly wished for, but now 

 more than ever to be desired, to test the penetrating theory of 

 a Russian Astronomer and Mathematician. 



C. Piazzi Smyth 



15, Royal Terrace, Edinburgh, January 19 



Iridescent Clouds 



On pp. 14S and 149 of the current volume of Nature there 

 are two letters describing " iridescent clouds," and the idea is 

 conveyed that this phenomenon is only of late occurrence. That 

 this is hardly justifiable, the following account from a diary will 

 show : — 



At Knoxville, Tennessee, on the afternoon of February t6, 

 1878, after many days of cloud and drizzle — something unusual 

 in that country — the sun being about 10" high and the sky par- 

 tially covered with large haze-clouds, there was noticed in the 

 south-west, against one of these clouds, a slightly curved band 

 of prismatic colours about 90° in length ; which, but for its posi- 

 tion in the west, might have been mistaken for a rainbow — concave 

 toward the sun, the sun, however, not at the centre of curvature, 

 and about 30° distant from it. The green was most strongly 

 marked ; this shaded off on each side, and on the side of the band 

 next the sun was red ; upon the opposite side the colour was less 

 distinct, but there it seemed to be-reddish. 



Again, during September or October 18S2 (this from memory), 

 at the same place, about sunset, with a patched, cloudy sky, the 

 sun not visible, the prismatic colours were noticed in the south- 

 west near a break in the clouds. This time the colours were in 

 the form of an elongated ellipse, with indistinct edges, between 

 2° and 3 s in greatest length. 



Then, during the fall of 1SS3, the prisoiatic colours were once 

 noticed under similar circumstances to those mentioned here in 

 Virginia. W. G. Brown 



University of Virginia, Virginia, U.S.A., January 12 



The Iridescent Clouds alluded to above 

 In our northern as well as insular position, with weakened 

 sunshine and an atmosphere always more or less darkened by 

 coal-smoke, we must be prepared to allow much for what is, and 

 is to be, seen of the grander meteors of meteorology in the more 

 southern latitude, clearer air, and intensified climate of the 

 Virginian portion of so great a continent as America. But 

 before any one there can claim to see frequently that very phe- 

 nomenon of the iridescent clouds, communicated last December 

 to Nature by various persons, but by myself perhaps as the 

 chief culprit, he must be quite sure, amid the crowd of known 

 and already described parhelia, mock suns, broken rainbows, 

 &c, that what he sees has the same discriminating optical cha- 

 racteristics as those particular clouds now in question ; and that 

 any one of his cases was, in America, so unusually brilliant a 

 display of them, and so widespread an instance of it, that from 

 one end of the States to the other it was on the same day simi- 

 larly seen, wondered at, and declared even by gray-headed old 

 men to be new to them, in at least anything approaching that 

 astonishing degree of splendour and perfection, though by no 

 means new to creation over a longer lapse of time. 



The Virginian letter-writer, however, speaks merely of what 

 he himself saw, describes the colours as prismatic, in place of 

 the anti prismatic arrangement witnessed here, and alludes to 

 one case of a curved band "about 90 in length," which con- 

 trasts exceedingly with the forms and sizes noted in this 

 country. 



