130 Sir J. F. W. Herschel on the Action of the Rays 
by mere continuance of illumination,which has excited surprise 
in some, but which is a perfectly natural consequence of this 
view of the matter; while in the opacity or strong absorptive 
power of the film producing these tints, we see reason to con- 
clude that no increase of thickness which long exposure would 
give to it, would ever develope in it the complete succession 
of the Newtonian colours beyond the first or second order; 
and that in consequence the subsequent longer continued 
action of the light would fail to produce distinct alternations 
of positive and negative pictures, though it might give rise to 
fluctuations of intensity, ending ultimately however in general 
and total blackness. Many of the intricate phenomena de- 
scribed by Prof. Moser, in his papers above alluded to, will 
be found divested of much of their enigmatical character by 
these and similar considerations. 
I shall close what I have to say on the subject of Dr. 
Draper’s specimen, by mentioning that about the time when 
that specimen was produced*, I was myself engaged in form- 
ing Daguerreotype impressions of the spectrum. My expe- 
riments were made in the last week of July and the first two 
or three days of August 1842. Want of habitude in the ma- 
nipulations of the Daguerreotype process (which I had never 
before executed), and by no means want of sun, prevented my 
obtaining anything like so fine impressions ; but I remained 
satisfied at that time, and by those experiments,—1st, that 
the tints of the coloured portion C were those of the New- 
tonian reflected series, more or less modified by the high re- 
fractive power and opacity of the film; 2ndly, that the law of 
intensity of action in proceeding from end to end of the 
spectrum, on the Daguerreotype plate, so far as I observed it 
(for I didnot obtain the “ negative” terminal portions A or E), is 
identical with that exhibited on blackened argentine paper (free 
from nitrate) under iodic influence, and especially the place 
of the abrupt maximum, which is so characteristic of iodine 
as a photographic element, proved (by actual measurement) 
to be precisely the same in both ways of operating. ‘This 
identity, I should observe, is somewhat difficult to recognize 
in Dr. Draper’s plate, where the maximum would appear to 
occur somewhat lower on the spectrum; but this, if it be 
really a point of difference between us, and not merely owing 
to a different nomenclature of colours, or to the effect of in- 
clining the plate to the incident ray, may and probably has 
arisen, not from any difference in quality of sunshine in Vir- 
ginia and England, but from difference inthe law of photogra- 
phic dispersion in the prisms used. 
* The specimen bears date July 27, 1842. 
