Jtdy I, 1880] 



NATURE 



195 



any haze, fog, or smoke surface nearer still than the 

 clouds. 



3. These precautions being taken, there was no difficulty 

 in recognising,yfr5/, during frosty weather, when meteoro- 

 logists know there is a minimum of moisture in the air, 

 what should be the normal appearance of the dry-gas 

 lines or bands, for they only are then conspicuous, and 

 are chiefly great B, the alpha band between C and D, 

 and a remarkable band on the green side of the universally 

 known D line of the regular solar spectrum. That band 

 being remarkable, not only for being situated as a dark 

 shade in the otherwise brightest part of the spectrum of 

 daylight, but by being much more dependent than the 

 other dry bands, on the lowness of altitude of the sun at 

 the moment, for its full and darkest development, and 

 thence called in these inquiries "the low-sun band." 

 Next, in the summer season of the year when the tempe- 

 rature has risen say to 70°, and we know, as per the 

 acknowledged hygrometrical tables, that there is then four 

 times as much, to the eye invisible, moisture in the air, for 

 that reason only, as at 30° temperature — spectroscopy 

 observation will show, simultaneously with and in addition 

 to, all the previous dry-gas lines, not only a strong water- 

 gas, or vapour, line closely following C, a true sun-line, 

 but a much grander line, double line, or rather band of lines 

 immediately preceding the solar line D ; and this par- 

 ticular water-vapour group is in practice the only one of 

 that kind which meteorologists need attend to in their 

 ordinary daily work. 



So far indeed we have only got, by its means, a species 

 of thermometer ; but if we go on observing day after day 

 in nearly similar summer temperature, and accustom our- 

 selves thereby to the then quality of appearance of that 

 band preceding D — and if on the next day, at the same, 

 or nearly the same, temperature, we should see the band, 

 say twice as dark as on the previous days — then in that 

 excess of darkness it has become " the rain band" sought 

 for. Because that abnormal excess of darkness shows as 

 infallibly as though it were written up in the sky, that 

 there is at that moment far more invisible watery 

 vapour in the upper atmosphere than the air there is 

 capable of holding much longer in suspension, where- 

 fore such extra moisture must very shortly be deposited 

 as rain. 



This then is the "rain-band spectroscopy" established 

 by the Edinburgh experimenter ; and it may be now 

 successfully practised with the smallest spectroscopes 

 either at home or abroad ; when one is travelling as well 

 as when stationary, for it occupies only a moment of 

 time each day, as the merest glance will tell whether the 

 rain-band preceding D is much stronger or less strong 

 than the normal quantity ; and with all the more certainty 

 on account of the low-sun band immediately following D 

 in the spectrum, enabling a differential, as well as an 

 absolute, estimate of darkness to be formed. While in 

 any but very cold and wintry weather, when Nature 

 herself lilts the balance for rainfall by a very small 

 addition to the watery vapour in the air, the spectral 

 indications are easily read off and apprehended. They 

 have also been found as certain at sea, in South Africa, 

 and India, or wherever the system has been carried, 

 as in Great Britain during the best part of its summer 

 season. 



If the research be further prosecuted with large dis- 

 persions, high magnifying powers, and on the direct light 

 of the sun, the hazy bands above spoken of as existing in 

 the general daylight, are found in the same spectral 

 places, but breaking up into scores and hundreds of fine 

 lines ; while the range of visible spectrum then extending 

 from the B limit of the mere sky and indirect sun's light, 

 to great A and beyond it — the intermediate groups of 

 lines called "little a and its preliminary band" will be 

 found a still more powerful " rain-band ' ' than that near 

 D. For though they, little a and its preceding band, are 



composed of lines vanishingly thin, te'vf and far apart in 

 dr)'" weather and as seen in a high sky, yet their interstices 

 in damp weather become peopled with myriads and 

 myriads of black lines, so as at last, indeed, in a setting 

 sun, to block up the whole space of each group, from one 

 side to the other, solidly ; and then actually to dwarf the 

 almost proverbial spectral colossus "great A," into a 

 mere line comparatively puny and unimportant. 



The Red-End of the Satar Spectrum. — "There must 

 surely be some mystery of difficulty," thought the experi- 

 menter, "touching the. red-end of the spectrum of the 

 high sun, or the able Angstrom, of Upsala, would not 

 have omitted it in his otherwise grandly perfect normal 

 solar spectrum map ; while some points in the Royal 

 Society's Himalayan solar spectra would have been very 

 differently rendered." Now one undoubted obstacle to 

 mapping that part of the spectrum well, is undoubtedly 

 the faintness of its light ; wherefore an idea immediately 

 occurred to the new worker that the qualities of his aurora 

 spectroscope were the very desiderata for both the red, 

 the ultra-red, and for ever5thing, in fact, that the very 

 beginning of the solar spectrum has to show beyond, 

 or earlier than, the point where Angstrom's spectrum, 

 commences its admirable delineations with the rudimen- 

 tary lines of only little a and its preliminary band. 



But the summer of 1S77 threatening to be hopelessly 

 cloudy in Scotland, the experimenter, after considering 

 the various pros and cons of many southern stations, 

 decided on trying Lisbon, as the place of all others giving 

 the highest summer sun, the best climate, and most social 

 facilities, with least time lost in getting there. And then 

 he further happily experienced, not only that the mag- 

 nificent steamers of the Pacific Steam Navigation Com- 

 pany of Liverpool, are the most admirable m.eans of 

 accomplishing the ocean transit ; but that the directors 

 of that Company are most favourably and liberally 

 inclined to help on any really scientific matter when for 

 the sake of science alone. 



Hence it was that in June and July, in Portugal, with 

 his Edinburgh spectroscope and a heliostat of simple 

 construction worked for him by his Wife's hand and eye, 

 the experimenter was enabled to see and map at noon- 

 day, and, day after day, in a high sky, free from ordinary 

 telluric effects, all that portion of the solar spectrum 

 outside, or situated preliminarily to. Angstrom's shortened 

 beginning ; viz., the matchless series of lines that go 

 towards forming the colossal groupings of both great A 

 and its grand preliminary band ; then beyond that the 

 very strong line Y and the groups of finer lines on each 

 side of it ; and beyond that again, near the beginning of 

 all visible spectral light, the strong line X, and certain 

 thinner lines on either side of it. 



Photography, as Capt. W. de W. Abney has admirably 

 shown since then, can take account of many more lines 

 still ; and some of them so very far away beyond all 

 visible red light, as to remind one of those other Hnes 

 recorded by Dr. Draper, the elder, in his celebrated 

 Daguerreotype spectrum taken in 1S43, I'nes which are 

 quite outside the pale of all optical spectra. But the 

 human eye had probably never, up to 1S77, seen more in 

 the "red" than what the Edinburgh experirnenter's 

 rough'y built-up aurora spectroscope showed on this occa- 

 sion : 'and the whole result, as contributing at last the 

 head-piece required for Angstrom's normal solar spec- 

 trum, was described in the 14th volume of the " Edinburgh 

 Astronomical Observations," published towards the end 

 of the same year 1S77. It was accompanied there by a 

 map, extending from the beginning of all visible light, 

 and including 62 lines up to the groups of little a, so 

 correctly represented in themselves by the philosopher 

 of Lfpsala. 



( To be continued.) 



