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sequently be drawn from the foregoing discussion of the Newtonian 
Physical Philosophy. 
(1.) Foremost among the results of Theory are to be placed the 
explanations of phenomena of Light, which are given by means of 
a mathematical investigation of the laws of Undulations of the 
Ether. These explanations, so far as they rest solely on the 
assumed properties of the ether, are analogous to explanations of 
phenomena of sound by vibrations of the air. So numerous and 
specific, and so complete, are the theoretical explanations ot this 
class, that in published discussions of them I have ventured to 
express the opinion that of themselves they afford strong pre- 
sumptive evidence not only of the reality of the ether, but also ot 
its being such as it was assumed to be. 
(2.) The mathematical investigation of the motions ot the 
ether conducts to a unique species of motion which may be called 
ray-vibration, inasmuch as it consists of vibratory motions partly 
transverse and partly parallel to an axis. These ray-vibrations 
may be supposed to be the exponents of rays of light ; and whereas 
experiment has shown that the sensation ot. light results from 
action on the retina of the eye, the direction of which is transverse 
to the direction of incidence of the light, it is reasonable to ascribe 
this action to the transverse vibrations above mentioned, and to 
suppose the direct vibrations to be in this respect inoperative. 
(3.) The phenomena of Light which depend on relations between 
atoms and the motions of the ether are not in general as readily 
explained, on account of our not knowing the exact conditions of 
the problems, as those which depend simply on the motions ot the 
ether. But in cases in which the atoms are constituents of 
regularly crystallized substances, phenomena under special circum- 
stances have been observed which admit ot satisfactory explanation 
on the Undulatory Theory of Light. It is here that the Theory 
of Lio-lit is brought into relation with the sciences of mineralogy 
and crystallography. 
(4.) The sensation of Light, as well as the phenomena of Light 
generally, being attributable to motions of the etlici which arc of 
the first order, "and consequently vibratory, or recurring motions, it 
follows that the transverse light-producing vibrations impress on 
atoms only vibratory motions. But I have found, by including in 
the mathematical calculation terms of the second order, that the 
direct vibrations of a ray, or those of a wave composed of an un- 
limited number of ray-vibrations, are capable of causing an atom 
to vibrate, and, at the same time, giving it a permanent motion of 
translation ; and that even the transverse vibrations can produce 
the same effect. There is nothing antecedently improbable in this 
result ; for, in fact, it has recently been ascertained by experiment 
