of the Solar Spectrum on the Daguerreotype Plate. 121 
in such inquiries, as a substitute for such analysis, in the pre- 
sent state of our knowledge of the absorptive powers of such 
glasses, serves only to confuse and mislead. In illustration of 
this proposition I need only refer to the statements of Pro- 
fessor Moser* as to the action of the green and yellow rays on 
the iodide of silver, in his papers on the process of vision and on 
invisible light ; statements which, embodying, at least in inten- 
tion, the results of elaborate and no doubt most carefully con- 
ducted experiments, are rendered perfectly unintelligible to 
one who has studied the subject with the aid of the prism, 
by his continual (and as it would almost appear systematic) 
substitution of the apparent (or absorptive) colours of glasses 
for the prismatic colours of rays, which he appears to assume 
that such glasses insulate in a state at least approaching to 
purity; an assumption unwarranted by the whole tenor of the 
phzenomena of absorption, and in the case of yellow glasses 
most particularly open to objection, as any one may readily 
satisfy himself by looking through such a glass at a prismatic 
spectrum, and by referring to Art. 103. of t my paper * On the 
Chemical Action of the Rays of the Solar Spectrum,” &c. 
(Phil. Trans., 1840), and repeating the experiments there de- 
scribed. 
2. Professor Draper’s specimen consists of a Daguerreotype 
silver plate, about 34 inches by 3 inches, on which is exhibited 
the impress of the spectrum in the form of a streak 3°3 inches, 
or thereabouts in length, and 0°08 inch in breadth, the edges 
being perfectly rectilinear and sharply defined throughout the 
whole length until within about a third of an inch from either 
termination, where they curve into an elliptic form so as to 
terminate the impression with two very elongated semiellipses, 
which are also very faintly and feebly marked. This, together 
with the very high proportion of 41 to 1 between the length 
and breadth’ of the photographic spectrum, sufficiently indi- 
cates it to have been formed by a non-achromatic lens, in- 
clining the surface of the plate forward so as to bring that 
part on which the more refrangible rays fall nearer the lens 
than that whici receives the less, thus compensating the short- 
ening of the focus for the former rays. And as such (though 
not stated in respect of this individual specimen) appears to 
have been Dr. Draper’s usual practice; and as in another of 
his papers he formally recommends it as securing *‘ the great 
aisonineeo of elongating the total length of the speett um, ; and 
though he seems to have read with aay cursory attention what had al- 
ready been published on this subject in England.—Note added during the 
printing. 
* See ‘Translation in Scientific Memoirs, part 11. (vol. iii.), p.422,—Eb. 
