RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 27 
senting some particular preparations to the action of the spectrum. And to begin 
with the simplest case, that of the nitrate of silver alone. 
72. Nitrate of Silver. (Paper No. 570.*). — Smooth wove demy paper washed with 
a solution, specific gravity 1T32. The general colour of the spectrum is a rather 
pale brown inclining to pink, and the tint is nearly uniform, except towards the most 
intense part (which falls about the middle of the blue rays), where it somewhat in- 
clines to a dark grey or lead colour, while, on the other hand, at the violet extremity 
of the spectrum it more inclines to pink. The total length of the spectrum impressed 
on white paper, as well as the insensible gradation of its most refracted end will ad- 
mit of measure, is about eighty-five parts. Its least refracted termination (at which 
it shows no sign of red, green, or blue tints) is at — 3% which allowing 3’6 for the 
sun’s semidiameter, may be considered as placing the limit of action almost exactly 
at the fiducial point. The maximum of intensity occurs at + 23 parts, or there- 
abouts ; and there is a considerable indication of a second maximum at about -J- 43 
parts, after which it degrades very rapidly, as we advance into that region of the 
spectrum which we have designated as the lavender rays. The feebleness of action 
of these rays, as compared with that of the blue and violet, is indeed the chief cha- 
racteristic of this spectrum. 
73. When the spectrum is made to act on paper similarly prepared, but previously 
discoloured by exposure to sunshine under cupro-sulphate of ammonia, the pheno- 
mena are materially different. The photographic spectrum is lengthened out on the 
red or negative side by a faint but very visible red portion, which extends to — 13*7^ 
or fully up to the end of the red rays as seen by the naked eye. The tint of the ge- 
neral spectrum, too, instead of brown is dark grey, passing, however, at its most re- 
fracted (or positive) end into a ruddy brown. The maxima occur nearly at the same 
points as above stated, but that at the end of the violet is more unequivocal, and the 
degradation beyond it more sudden. 
74. Muriate of Sodaf. (Papers Nos. 545, 559, 568.) — The spectrum is very long, 
intense, and far more variously coloured than any other which has hitherto occurred 
in my experiments. If the paper be purely white and the day cloudless, the visible 
impression commences with a pretty high red at — 7*6, which at about half that 
distance ( — 3'8) below the fiducial point begins to pass into green, through a kind 
of livid mixed tint. The best green, which is, however, of a sombre and dull cha- 
racter, is developed at or a little above the fiducial point, covering a breadth of about 
four parts. Thence, with a barely perceptible tinge of dark blue, it passes rapidly 
into intense blackness, which at -j- 80 begins to die away into a purplish brown, and 
* These numbers refer to memoranda kept of the preparation of the various papers used. 
t When one ingredient only is thus mentioned, it is understood that that ingredient has been applied as an 
intermediate wash between two washes of nitrate of silver; thus the preparation of the paper No. 559, on re- 
xerence to the memoranda kept, appears to have been, 1st wash, nitrate of silver 1’132 ; 2nd ditto, muriate of 
soda, 1 salt + 19 water; 3rd ditto, nitrate of silver 1'096. If the word lead be added, it is to be understood 
that the first wash has been acetate of lead. 
E 2 
