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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 observations which 

 may facilitate the understanding of the main object of the inquiry. 



All impulses, says the author in the First Proposition, are propa- 

 gated 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 different 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 effect 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 ah* 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 undulatory 

 system. The subject of this Proposition having always been con- 

 sidered as the most difficult 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 phenomena of Refraction, Reflection, and 

 Colours. They are stated in the following manner. When an un- 

 dulation arrives at a surface which is the limit of media of different 

 densities, a partial reflection takes place, proportionate in force to 



