of the Solar Spectrum on the Daguerreotype Plate. 123 
nestly recommend the subject to our mathematicians and 
artists as highly deserving their attention. 
3. But to return to Dr. Draper’s specimen. The ground 
on which it is projected is dimmed by the vapours of mercury 
settling on the parts where dispersed light has fallen, so as to 
be rendered much less specular, not only than the unattacked 
part of the silver surface, but also than the impression of the 
spectrum itself, which for the most part yields a much more 
powerful and regular reflexion than any part of the contiguous 
ground. 
4. With respect to the phenomena offered by the ground, 
it is that of the Daguerrotype in general, and being produced 
by mized rays, I shall defer its consideration, remarking only at 
present, that it appears dark in certain lights, white in others, 
and iridescent with a faint halo-like pink tint in the transition 
from one light to the others, especially towards its borders. 
5. The spectrum itself is extremely remarkable and beau- 
tiful. It is divided, on a superficial inspection, into several 
very distinct compartments, which however, with one excep- 
tion, are found on close examination to graduate imper- 
ceptibly into each other, though with so high a degree of 
abruptness of transition as may justify our regarding it, in 
description, as consisting of differently characterized regions. 
The apparent outlines of these regions are represented in the 
figure annexed (Plate II. fig. 1), though only a coloured draw- 
ing could properly render the general effect and delicate gra- 
duation of the several regions (indicated by the letters) into 
each other. 
6. The aspects of the spectrum, as has been observed of 
the ground, are very different in different lights, assuming the 
one or the other of two opposite characters, according as it is 
viewed by specular reflexion or by side light. ‘These aspects it 
will be necessary to describe separately. 
7. When viewed by specular reflexion, as when viewed at 
a nearly perpendicular incidence with the back to an open 
window, or when laid on a table by candle-light and the 
ground glass or paper shade of a lamp seen clearly by re- 
flexion on the general surface, while no side light is suffered 
to fall on it; the ground, as above observed, is dark in compa- 
rison with the unattacked silver surface. On this ground the 
terminal regions A, E appear bright and specularly white, 
though materially less so than the unattacked silver. The 
region B is black, though not absolutely so: fully as dark 
however as the ground. ‘This is immediately followed by C, 
which is white at its outer edges, but very gradually passes in- 
wardly into pale yellow, yellow, and at last terminates in an oval 
