REPORT ON OPTICS. 
321 
example, examine the action of absorbent media on any one of 
the 590 rays which are deficient; and there is no possible way 
of recognising them, for the purpose of examination, in the spec¬ 
tra of those white artificial flames where they all exist. 
This difficulty, however, has been completely removed by the 
discovery which I have lately made, of a gaseous substance 
which produces more than a thousand dark lines in the spec¬ 
trum of ordinary flames, and thus renders artificial light more 
valuable even than that of the sun for the determination of op¬ 
tical data, while it enables us to study the action of material 
bodies upon all the defective rays of solar light. I have men¬ 
tioned this experiment at present, in order to point out its bear¬ 
ings upon the two rival theories of light. On the Newtonian hy¬ 
pothesis of emission, the fact may be thus stated:—When a 
beam of white light is transmitted through a certain thickness 
of a particular gas, a thousand different portions of that beam 
are stopped in their passage, in consequence of a specific action 
exerted upon them by the material atoms of the gas,—an action 
which is powerfully assisted by the simple application of heat. 
Such a specific affinity between definite atoms and definite rays, 
though we do not understand its nature, is yet perfectly con¬ 
ceivable; and we may render it more easy of reception by hazard¬ 
ing the conjecture, that the particles of light itself are identical 
with the ultimate molecules of bodies, and that similar atoms in 
each may again unite when brought within the spheres of their 
mutual attractions. 
In the language of the undulatory theory, the same fact may 
be thus expressed. A thousand different waves or rays of light 
of different velocities or refrangibilities, are incapable of propa¬ 
gating undulations through the aether of a transparent gas, 
while all waves or rays of intermediate velocities and refrangi¬ 
bilities are freely transmitted through the same medium: that 
is, a wave of red light, the 250 millionths of an inch broad, 
and another wave of the same light the 252 millionths of 
an inch broad, are capable of transmitting vibrations freely 
through the gas, while another red ray the 251 millionths of 
an inch produces vibrations which are entirely stopped by 
the medium. There is no fact analogous to this in the phe¬ 
nomena of sound, and I can form no conception of a simple 
elastic medium so modified by the particles of the body which 
contains it, as to make such an extraordinary selection of the 
undulations which it stops or transmits. We may suppose, 
indeed, that ether is a compound medium, consisting of other 
media, whose particles are the ultimate atoms of matter, and 
that the undulations of the same ether in transparent bodies are 
somehow affected by the affinity of similar atoms in the ether 
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