SCO LECTURE XXXIX. 



effects ; and there is reason to conclude from experiments, that such a 

 force, if it existed, must extend to a very considerable distance from tfie 

 surfaces concerned, at least a quarter of an inch, and perhaps much more, 

 which is a condition not easily reconciled with other phenomena. In the 

 Huygehian system of undulation, this divergence or diffraction is illus- 

 trated by a comparison with the motions of waves of water and of sound, 

 both of which diverge when they are admitted into a wide space through 

 an aperture, so much indeed that it has usually been considered as an ob- 

 jection to this opinion, that the rays of light do not diverge in the degree 

 that would be expected if they were analogous to the waves of water. 'But 

 as it has been remarked by Newton,* that the pulses of sound diverge less 

 than the waves of water, so it may fairly be inferred, that '^ae- ^till more 

 highly elastic medium, the undulations, constituting light, must diverge 

 much less considerably than either. (Plate XX. Fig. 26G.) 



With respect, however, to the transmission of light through perfectly 

 transparent mediums of considerable density, the system of emanation 

 labours under some difficulties. It is not to be supposed that the particles 

 of light can perforate with freedom the ultimate atoms of matter, which 

 compose a substance of any kind ; they must, therefore, be admitted in all 

 directions through the pores or interstices of those atoms ; for if we allow 

 such suppositions as Boscovich's, that matter itself is penetrable, that is, 

 immaterial, it is almost useless to argue the question further. It is cer- 

 tain that some substances retain all their properties when they are reduced 

 to the thickness of the ten millionth of an inch at most, and we cannot there- 

 fore suppose the distances of the atoms of matter in general to be so great 

 as the hundred millionth of an inch. Now if ten feet t)f the most trans- 

 parent water transmits, without interruption, one half of the light that enters 

 it, each section or stratum of the thickness of one of these pores of matter 

 must intercept only about one twenty thousand millionth, and so much must 

 the space or area occupied by the particles be smaller than the interstices 

 between them, and the diameter of each atom must be less than the hun- 

 dred and forty thousandth part of its distance from the neighbouring par- 

 ticles ; so that the whole space occupied by the substance must be as little 

 filled as the whole of England would be filled by a hundred men, placed at 

 the distance of about thirty miles from each other. This astonishing 

 degree of porosity is not indeed absolutely inadmissible, and there are 

 many reasons for believing the statement to agree in some measure with 

 the actual constitution of material substances ; but the Huygenian hypo- 

 thesis does not require the disproportion to be by any means so great, since 

 the general direction and even the intensity of an undulation would be 

 very little affected by the interposition of the atoms of matter, while these 

 atoms may at the same time be supposed to assist in the transmission of 

 the impulse, by propagating it through their own substance. Euler indeed 

 imagined that the undulations of light might be transmitted through the 

 gross substance of material bodies alone, precisely in the same manner as 

 sound is propagated ; but this supposition is for many reasons inadmis-' 

 sible. 



* Op. Qu. 28. 



