LINE Y, IN THE INFRA-PLED OF THE SOLAR SPECTRUM. 235 



black velvet, in order to exclude any reflected light ; a low power was em- 

 ployed; the slit was made about the eighth, or tenth, of an inch wide,* and 

 the eye of the observer was washed with water to cleanse the fluid that lubri- 

 cates the cornea. The most prominent line in this space is that marked Y." 



That last remark is quite to our purpose, and I trust the drawing-strip, 

 No. 1, of the Plate now given, illustrates it perfectly, remembering that " great 

 A" and "little a" are introduced merely to give milestone references to known 

 parts of the spectrum, and a measuring test universally understood for scale. 



Strip No. 2 represents some rude efforts of mine in 1871, with very unequal 

 apparatus, to see something of this rare region of the ultra-red. The drawing 

 is slightly altered from that in Vol. XIII. of the Edinburgh Astronomical Obser- 

 vations, inasmuch as the mere general appearances of many close, thin lines 

 unmeasured, and of shading, improperly represented there by vertical lines, are 

 here crossed diagonally and horizontally in such a manner that they cannot be 

 understood to imply true, resolved spectral lines, or anything but shade only, 

 symbolically expressed. And the chief result is thereby plainer than ever, viz., 

 that the Y line was better seen in a high summer, than a low winter, sun ; a 

 feature indicating it to be of Solar origin, and not of Earth's atmosphere, or 

 " Telluric " intervention. 



Strip No. 3 gives the two views of high and low sun, contained in the Royal 

 Society's Himalaya spectrum, in their Philosophical Transactions for 1875. 

 This drawing is on a smaller scale than their's ; and their questionable shadings 

 with vertical lines have been changed by me into diagonal lines ; but otherwise 

 it represents in exactly the same manner their very surprising negation of the 

 visibility of Y in a high sun, but its abundant visibility, and that of Brewster's 

 Z also, in a low sun. 



Strip No. 4 represents on a reduced scale my own observations (from 

 Vol. XIV. of Ed. Ast. Obs.) made in Portugal in 1877, with a far more powerful 

 spectroscope than I had ever possessed before, and which I had had constructed 

 specially to look into this particular question of the visibility, or non-visibility, 

 of the Y line in a very high, indeed almost Zenithal, sun. The result, as will 

 be seen in the drawing, was to confirm the previous Edinburgh observation, and 

 to show that Y was, with the sun near the zenith, most notably visible ; Brew- 

 ster's X appearing next in strength ; but Z only in the faintest manner possible, 

 if at all. 



Strip No. 5 is a very reduced copy of part of a magnificent work derived 

 from photography by Captain Abney and Colonel Festing, forming the 

 Bakerian Lecture at the Royal Society for 1880. 



* The distance of this slit is unfortunately not stated. It may have been at the other end of a 

 long room, and was apparently unfurnished with any kind of collimator lens, in the improved manner 

 introduced by Professor Swan. 



