missibility. It assumes that all material bodies have an attraction
for the ethereal medium, by means of which it is accumulated within
their substance, and for a small distance around them, in a state of
greater density, but not of greater elasticity.
The whole theory is now applied to the phenomena, in Nine Pro- positions, together with several scholia and corollaries. As the argu- ments on which the doctrine rests cannot be abbreviated Without impairing their evidence, we must content ourselves with merely enumerating the heads, adding, however, a few olmervations which may facilitate the understanding of the main object of the inquiry.
All impulses, says the author in the First Proposition, are propagated in a homogeneous elastic medium, with an equable velocity. The truth of this theorem has been mathematically demonstrated by various writers, the actual motion being considered as very minute. Prop. 2. An undulation conceived to originate from the vibration of a single particle must expand through a homogeneous medium in a spherical form, but with différent quantities of motion in different parts. Prop. 3. A portion of a spherical undulation, admitted through an aperture into a quiescent medium, will proceed to be further pro- pagated rectilineally in concentric superficies, terminated laterally by weak and irregular portions of newly-diverging undulations. The chief purport of this last Proposition is to obviate the objection, that if light were the efi’ect of a widely-expanded fluid put in motion by an impulse, it would, like the waves of water, and air in the instance of sound, spread laterally from the original direction of the motion, and agitate the contiguous quiescent medium; by which means we ought to see objects as well as we hear sounds, behind an opaque body. It is here alleged that, according to Newton’s own words, sounds diverge much less than the waves of water, the air being more rare and elastic; and that in the very rare luminous medium, after its undulations have passed by an opaque substance, they will indeed diverge a little from their first direction, but this in so small a de- gree as to be almost imperceptible; whereas the loss of even this small degree of impulse will make the progressive undulatory beam appear somewhat contracted. It is no small confirmation of the theory, that this effect perfectly agrees with the result of an experi- ment mentioned by Sir Isaac Newton; though, having adopted a dif- ferent principle, he used it rather as an objection to the nndulatory system. The subject of this Proposition having always been con- sidered as the most diflicult part of the last-mentioned system, the author has taken particular pains to clear it as much as possible from all objections.
The mere recital of the enunciations of the four next Propositions will probably enable those at all conversant with the science of optics, to perceive in what manner the author means to explain, according to his theory, the important phaenomena of Refraction, Reflection, and Colours. They are stated in the following manner.—When an undulation arrives at a surface which is the limit of media of different densities, a partial reflection takes place, proportionate in force to
