458 LECTURE XXXIX. 



We shall proceed to examine in detail the manner in which the two principal 

 hypotheses respecting light may be applied to its various properties and 

 affections; and in the first place to the simple propagation of light in right 

 lines through a vacuum, or a very rare homogeneous medium. In this 

 circumstance there is nothing inconsistent with either hypothesis; but it 

 undergoes some modifications, which require to be noticed, when a portion 

 of light is admitted through an aperture, and spreads itself in a slight 

 degree in every direction. In this case it is maintained by Newton that the 

 margin of the aperture possesses an attractive force, which is capable of in- 

 flecting the rays: but there is some improbability in supposing that bodies 

 of different forms and of various refractive powers should possess an equal force 

 of inflection, as they appear to do in the production of these effects; and there 

 is reason to 'conclude from experiments, that such a force, if it existed, must 

 extend to a very considerable distance from the 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 Iluygenian system of undulation, 

 this divergence or diffraction is illustrated by a comparison with the motions 

 of waves of water and of sound, both of which diverge when they are ad- 

 mitted into a wide space through an aperture, so much indeed that it has 

 usually been considered as an objection 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 in- 

 ferred, that in a still more highly elastic medium, the undulations, constituting 

 light, must diverge much less considerably than either. (Plate»XX. Fig. 266.) 



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 ligh 

 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 suppo- 

 sitions as Boscovich"s, that matter itself is penetrable, that is, immaterial, it h 

 almost useless to argue the question further. It is certain 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 therefore suppose the distances 



