126 Sir J. F. W. Herschel on the Action of the Rays 
Draper supposes, has been protected from the action of di- 
spersed light: and this is the difficulty which, as I have 
already stated, I do not wish to dissemble, and which Dr. 
Draper’s negative rays introduced at the ends of the spec- 
trum will not, I conceive, fully get over. The form of the 
curve, fig. 2, is fully in unison with what I have described 
and figuréd respecting the action of the spectrum on iodu- 
retted papers (Phil. Trans, 1842, Article 214—216). And if 
we have regard to the breadth as well as the length of the 
spectrum, its law of intensity will be represented by the ver- 
tical coordinate of a solid surface represented in front and 
lateral projection in figures 3, 4, and if each such coordinate 
be regarded as expressing the proportional thickness of the 
deposited film at that point, the solid itself will represent on 
a very exaggerated scale (of perhaps a million to one in 
height) the actual form of that film, supposed continuous. 
11 In this view of the subject the deposited film must be 
regarded as homogeneous. 7. ¢, one and the same chemical 
substance (whatever that substance be) in its whole extent: 
granting this, all is one phenomenon regulated by a mathema- 
tical expression into which a parameter varying only with the 
refrangibility of the ray, the time of its action and the in- 
tensity of the light, enters. And here I would observe, that 
the lateral gradation, which is of course due solely to varying 
intensity, is produced by two very distinct causes, both which 
it is well in such experiment to bear always in mind as ac- 
counting for various singular phenomena of internal ovals 
and reversed action along the axis of the spectrum as exhibited 
on photographic papers, &c. For, first, at the edges of the 
spectrum there is no overlapping of images, which overlapping 
from zero at that point goes on increasing to the axis. From 
this cause originates a progressive scale of intensity propor- 
tional to the chord of the sun’s circular image, measured in a 
direction parallel to the axis. And, secondly, it is placed 
beyond a doubt, both by Mr. Airy’s and my own observations, 
that the central portions of the sun’s disc are very much more 
luminous than the borders, which if it be due (as I have 
elsewhere stated I conceive it to be) to the absorption of a 
solar atmosphere extending beyond the luminous surface, 
would also act on the photographic powers of the rays un- 
equally, and so produce a variation in the lateral rate of pho- 
tographic action, of which at present we have no measure, but 
which may possibly be thus brought within our range of inves- 
tigation, and be rendered obvious by direct examination of the 
sun’s white image simply projected on photographic paper, an 
experiment I propose to try on a favourable occasion, 
