128 Sir J. F. W. Herschel on the Action of the Rays 
within and immediately adjoining to the luminous one. The 
thermic rays belonging to this region possess singular and 
remarkable properties, of which more presently. And if such 
conservation be in any way ascribable to thermographic op- 
position to photographic action, it seems not unreasonable to 
argue from the absence of observable white conjugate images 
on the discoloured ground in all the very numerous cases 
where it has been witnessed (with the single exception of that 
above noticed), that such action is limited to these peculiar 
thermic rays, and does not extend to the purely calorific ones 
remote from the spectrum. This subject, however, will re- 
quire more attention, which on the return of a summer sun I 
purpose to give it. 
To return, however, to Dr. Draper’s spectrum. The spe- 
cular colours of the portion C lower very perceptibly in the 
scale of tints when viewed in a very oblique incidence, a cir- 
cumstance agreeing with the analogy of the colours of thin 
plates. On the other hand, if the incident light be so managed 
as not to return by specular reflexion to the eye, but merely 
to illuminate the plate strongly, while all other specularly 
reflected light is eliminated by a proper adjustment of dark 
objects in the neighbourhood, the character of the phanomena 
undergoes a singular and striking reversal. The terminal 
regions A, E become dark, though less so than the unat- 
tacked silver; the half black space B becomes strongly white, 
and the violet-black space D very brilliantly so, showing like 
frosted silver; while on the other hand the coloured portion 
C becomes extremely dark, somewhat dull in its aspect, and 
of a hue which, so far as its great obscurity will permit it to 
be distinguished as a tint, may be termed steel-gray or bluish. 
By candle-light a tendency to green may be perceived. This 
tint, and the blackness it relieves, hardly undergo any varia- 
tion over the whole space C, except just where it graduates 
into whiteness at its union with B. 
In this succession we recognize, with very slight deviations, 
the series of Newtonian transmitted rays undiluted with the 
white light which accompanies them when formed by a film 
of air or glass, and (like those of the reflected series) advanced 
by the intrusion of a black member at their commencement. 
But it must be noticed as a fact of great moment in their 
theory, that they are not the complements of the tints seen by 
specular reflexion at the same points of the spectrum. In 
fact they go no higher than to one-half the extent of the series 
so seen, since the black ring of the first order of transmitted 
rings corresponds, not to the first black ring, but to the white 
of the first order, in the reflected series. Here is no doubt 
