RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 37 
trum, and a white spectrum is produced, extending in some cases (but always feebly) 
below the fiducial point, having its maximum of intensity at the junction of the blue 
and violet, and fading away gradually over the more refrangible region. If the dose 
be very strong, the whitening goes on rapidly even without the aid of light. 
97. A paper prepared with acetate of lead and then chloride of platina (neutralized 
by lime) was washed over, not with the pure hydriodate of potash, but with a mixed 
solution of this salt and tartrate of soda and potash. The action of the hydriodate 
was enfeebled to a greater degree than by the mere effect of dilution. It required to 
be excited by a second application of the liquid, and even then the darkened space, 
in lieu of extending to the negative extremity of the spectrum, broke off at the fiducial 
point. On applying, however, a third dose, the action was again excited and carried 
instantly down to and far beyond the extreme red. It has been already noticed 
(Art. 47.) that paper so prepared is insensible per se, and acquires a negative cha- 
racter under the influence of thehydriodic salts, the gradual coming on of which cha- 
racter for rays of less and less refrangibility as the dose of the saline application in- 
creases, it is the object of this experiment to trace. 
98. In some instances, where, under the influence of the hydriodate, the paper has 
been rendered positive (i. e. whitened by light) for the violet and ultra-violet rays, 
and negative for the rest of the spectrum down to the red, another change of cha- 
racter from negative to positive has been observed to take place beyond the red, in- 
dicated by a slight but unequivocal whiteness produced about the region of the ex- 
treme red. This singular alternation of character, it may be as well to mention, oc- 
curred in two papers (Nos. 649, 656.), the one prepared with phosphate of soda, the 
other with muriate of strontia, both with a mordant base of lead ; but it is probably 
not so much dependent on the peculiar preparation of the paper as on some acci- 
dental adjustment of the dose of the iodic salt in the experiments referred to. 
99. In one experiment only was any other colour than mere black or dark brown, 
and white (or rather pale greyish yellow), observed to be produced under the influence 
of the hydriodic application. The paper in this case (No. 672.) had been prepared 
with muriate of lime and nitrate of silver without a mordant wash, and, when dark- 
ened by previous exposure and then subjected to the joint action of the spectrum and 
the iodic solution, was found to be negative in its character for rays more refrangible 
than the yellow, while in the less refrangible region it was strongly reddened. It 
should be observed, however, that in paper so prepared, and without the hydriodic 
application, the simple action of the spectrum developes an unusually strong red at the 
point appropriate to that colour. 
VI. Of the Analysis of the Chemical Rays of the Spectrum by Absorbent Media. 
100. As the effectual study of mixed colours, as nature presents them to us, would 
be impracticable by the mere use of absorbent media, unaided by prismatic analysis, 
so that of the photographic relations either of argentine or any other class of re-agents 
