RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 
45 
1. Saxon topaz, parallel to the axis. 1 These plates had a very slight purplish 
2. Saxon topaz, perpendicular to the axis, j tinge. 
3. Sulphate of lime. 
4. Iceland spar, a natural rhomboid. 
5. Plate glass, slightly tinged with blueish green. 
6. Rochelle salt, parallel to the axis (not very well polished). 
7. Thick quartz, perpendicular to the axis. 
8. Thin quartz, perpendicular to the axis. 
9. Thin quartz, oblique to the axis. 
10. Thin quartz, differently oblique to the axis. 
After exposure to the sun for a sufficient length of time to darken the ground of the 
paper considerably, it was found on taking off the plates and fixing the whole with 
hyposulphite of soda, that all the substances employed had considerably exalted the 
solar action, though not all in the same degree. The plate-glass, which, as observed, 
was a little coloured with blueish green, had produced decidedly the greatest effect, 
the discoloration beneath it being at least three or four times that of the general 
ground of the paper. The order of the others was as follows : 
Nos. 7,4 . . . . 6,8,3,10 . . . . 9,2,1, 
which, on the whole, was nearly that of the thicknesses of the plates, and does not 
seem, except in the case of the glass, to indicate any peculiar action in the media ; 
and in this case the slight tinge of green, by stopping the chemical rays beyond the 
red-f-, may be supposed to have increased the action. 
117 . At the same time that this experiment was made another circumstance was 
noticed, which added to the obscurity of the subject. A specimen of paper, infinitely 
more sensitive than the nitrated paper I at that time used, was sent me by Mr. Tal- 
bot. This paper, treated exactly in the same manner, and exposed at the same time 
with a specimen of the nitrated paper, exhibited no such exalting effect. This in- 
duced me to subject, at various periods, to similar trials a great number of prepara- 
tions and several varieties of paper, the results of which appeared exceedingly capri- 
cious, as in some cases it was found that exactly the reverse effect was produced, and 
that the superposition of one and the same glass in several instances exercised quite 
as remarkable an influence in depressing, as in others it did in exalting the solar ac- 
tion ; while in others, again, it seemed to exercise no influence at all. The quality 
of the paper, too, appeared to exercise a marked influence on the result. And dif- 
ferences in the mode of preparation, and the doses of the ingredients, too minute to 
be noted, would alter materially the degree, and sometimes even the character of the 
effect. The followingTable contains several of the results of these trials. Column 1. 
contains the number of the paper ; column 2. the character of the effect, in which -f- 
indicates an exalting influence on the glass, 0 indifference, and — a depressing in- 
* See Art. 77. 
