38 
SIR J. F. W. HERSCHEL ON THE CHEMICAL ACTION OF THE 
would be equally unavailing, were such media alone resorted to for the purpose of 
effecting a separation of the luminous elements prior to their action. A combination 
of their analysis with that effected by the prism will lead, however, to results of no 
small interest. The facts I am about to recite, so far from exhausting or even sy- 
stematically arranging and attacking the subject, are hardly more than sufficient to 
show what a singularly wide field of inquiry is here laid open to us. 
101. To study this subject in its most simple form, it would be necessary to ope- 
rate on some preparation which shall be equally and indifferently sensitive to every 
ray of the spectrum. Such a preparation, with exception of the rays beyond the ex- 
treme red, exists in the bromuretted paper of Art. 77- But when the paper used to 
receive the spectrum, after undergoing absorptive ahalysis, has its own peculiar law 
of sensitiveness, it is evident that the resulting impression left on such paper will be 
a mixed effect depending on two distinct functions, the one expressing the degree of 
transmissibility or diacratic index of the medium for that ray, the other that of sensi- 
bility in the paper to the action of such ray. 
102. I have already stated that I saw no reason from experiments made with prisms 
of water, crown- and flint-glass, to suspect any considerable difference in those media 
as to their respective scales of diacratescence* ; but it would be hasty on the strength 
of those experiments to affirm that none exists, and quite unwarranted to extend the 
conclusion to other colourless media. On such it was my design to have instituted 
a series of experiments expressly directed to this object ; but the want of sun ren- 
dered it impracticable, since it is useless, or only tends to confuse and mislead, to 
pursue such inquiries without the use of the spectrum ; and the various and striking 
phenomena exhibited by coloured media naturally gave them a precedence in point of 
interest. Some of the more remarkable of those which have fallen under my obser- 
vation I shall now proceed to describe. 
103. Red, brown, and yellow media are well known to exert their greatest ab- 
sorbent energy on the more refrangible rays ; and the progress from the palest yellow 
to the deepest red is marked by a corresponding disappearance in the spectrum, as 
transmitted by or viewed through such media, of these rays progressively, commencing 
with the violet and proceeding downward through the spectrum till only the extreme 
red is left. The various shades of brown seem to depend chiefly on the proportions 
in which the green rays are attacked in the progress of this general destruction. 
Nothing, however, which the eye can seize in the character of such media, in the or- 
dinary mode of examination, indicates any tendency towards an increase of transmis- 
sibility in the ultimate violet, or anything to authorize a suspicion that rays beyond 
the violet would not be cut off by such media, even more energetically than the visible 
violet rays themselves. It is true that by those red media, whose tint verges to a rose 
colour or pink, both blue and indigo rays are transmitted pretty copiously ; but the 
* Perhaps diacrasy would be preferable to a term confusing, as diacratescence does, a Greek and Latin 
origin. 
