152 
SIR DAVID BREWSTER A^TD DR. J. H. GLADSTO^T: OX 
no fluorescence is produced in bisulphate of quinine, or similar bodies ; while this repre- 
sents those where there is no efiect on the organ of vision. Professor Stokes mentions 
in a note to his paper, that he had “ succeeded, by a particular an’angement, in seeing so 
far into -the ‘ lavender ’ rays as to make out the groups of fixed lines m, by means 
of light received directly into the eye, and even to perceive hght beyond that.” The 
TO, n of his drawing answer to the M and N of M. Becqueeel’s map, and his p is nearly 
coincident with its termination. The figure of M. Matthiessex extends at least as far 
as the fluorescent spectrum of Professor Stokes, but it does not clearly appear how it 
has been obtained. 
Professor Piazzi Smyth, during his recent astronomical experiment on the Peak of 
Tenerifie, had an opportunity of analysing the light of the sun when seen through a 
smaller amount of atmosphere than has fallen to the lot of any other investigator. He 
has incorporated such observations made near noon-day in the accormt of his experi- 
ment but unfortunately his apparatus was imperfect. His statements amount to this : 
that “ comparing a high sun-spectrum at the sea-level with a similar one at Alta Vista 
10,702 feet high, both observed direct, it was found that while in the former the 
spectrum terminated immediately beyond H, and the two bars of H were nebulous, in 
the latter the spectrum extended beyond H to three times the distance of its bar's 
asunder: the two said bars also lost all theh nebulosity, being clearly resolved irrto 
their component lines ; many fine clear lines were seen between them, and many 
appeared nebulously in the space beyond.” He gives other good reasons also for the 
conclusion that “ we may assume that there is a much greater amount of the more 
refrangible rays in the sun’s light irr the upper, than irr the low'er regions of the atmo- 
sphere.” To his drawings of the spectrum at evening, refererrce will be hereafter made. 
Absorption by the Atmosphere. 
The absorbent power exercised by the earth’s atmosphere on the more refrangible 
rays already alluded to, is extremely evident when the sun is shining through a long 
reach of air, as at his rising or setting. Everyone has remarked that under such 
circumstances the luminary acquires a more or less red colour. This is due to the 
total absorption of the lavender and violet rays, the greater or less absorption of the 
blue and green, and the diminution of the yellow and perhaps the orange rays, whilst 
the red pass apparently unimpeded through the deepening atmosphere, and from the 
absence of the dazzling yellow, reveal themselves with their characteristic lines to the 
eye even when unshielded by any coloured medium. This is also the case, to a certain 
extent, when the sun at any altitude shines through mist or smoke ; and the same result 
may be at any time obtained by placing in the path of the rays a vessel of water udth 
which a few drops of milk have been mixed. 
It is scarcely necessary to say, that, though the Hsible spectrum becomes shorter as the 
* Philosophical Transactions, 1858, pp. 503 to 507. 
