RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 29 
Extremity of the white portion — 26-27 
Sun’s semidiameter + 3 - 60 
Extremity of the chemical action in a linear spectrum or centre of the 4 
. , 7-7 > ~ 22*67 
solar image m the extreme chemical ray J 
Centre of the solar image in the extreme red ray — 13-30 
Absolute extent of the chemical beyond the luminous spectrum . . — 9"37 
Commencement of the dull red fringe — 15-05 
Semidiameter of the sun + 3-60 
Point in a linear spectrum to which the darkening influenec would extend — 1 1*45 
Most refrangible extremity of the darkened portion + 90-50 
Total length of the darkened portion 105-55 
Entire length of the paper visibly affected 1 1677 
78. We may now observe, that this highly remarkable conclusion is also substan- 
tiated by the measurement stated in Art. 75- of the limit of whiteness on the inuri- 
ated paper; for if from — 22-43, the coordinate of the termination of the sensible im- 
pression, we subduct the sun’s semidiameter 3'6, we get — 18-83 for the limit of sensible 
chemical action on that paper in a linear spectrum* which already surpasses by —55 
the point (at — 13 - 3) at which the extreme red ray is situated in such a spectrum. 
79. Hydriodate of Potash. (No. 577-) — The preparation of this paper is very vari- 
able in its results according to the strength of the solutions used. If strong solution 
of the hydriodate be used, it is nearly or quite insensible ; if weak, the reverse. In 
the latter case the photographic spectrum it exhibits is not a little singular. It has 
the appearance of being inverted, the impression commencing very feebly and gradu- 
ally at a point nearly coincident with the mean red ray of the luminous spectrum, 
where however the trace impressed is barely perceptible. Thence it gradually and 
regularly increases upwards till it attains its maximum of intensity at about -f- 50, 
that is to say, about five parts beyond the extreme violet. It then degrades more 
rapidly than it increased, till it vanishes at + 9T2, giving a total range of ninety- 
six parts. The tints are also remarkable. There is no imitation of the colours of 
the spectrum itself, but rather a kind of inversion of them. For the least refrangible 
end is of a dull ash grey or lead colour, and the most so, a considerably ruddy snuff- 
brown ; the change of tint coming on rather suddenly about the end of the blue or 
beginning of the violet rays of the luminous spectrum. 
Possible indication of Absorptive Action in the Suns Atmosphere. 
80. Rochelle salt. Tartrate of Potash-and-soda. (Papers Nos. 573, 574.). — Of all 
the salts I have examined, this presents, perhaps, the most singular peculiarities. 
* By a linear spectrum I understand such a one as would be cast by a fixed star or other luminary of in- 
sensible angular diameter. 
