ON THE NATURE OF LIGHT AND COLOURS. 459 



of the atoms of matter in general to be so great as the hundred milllonjth of 

 an inch. Now if ten feet of the most transparent 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 dia- 

 meter of each atom must be less than the hundred and forty thousandth part 

 of its distance from the neighbouring particles: so that the whole space oc- 

 cupied by the substance must be as little filled, as the whole of England 

 would be filled l^y 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 state- 

 ment to agree in some measure with the actual constitution of material sub- 

 stances; but the Huygenian hypothesis does not require the disproportion to 

 be by any means so great, siuce 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 inadmissible. 



A very striking circumstance, respecting the propagation of light, is the 

 uniformity of its velocity in the same medium. According to the projectile 

 hypothesis, the force employed in the free emission of light must be about a 

 million million times as great as the force of gravity at the earth's surface; 

 and it must either act with equal intensity on all the particles of light, or 

 must impel some of them through a greater space than others, if its action 

 be less powerful, since the velocity is the same in all cases; for example, if 

 the projectile force is weaker with respect to red light than with 

 respect to violet light, it must continue its action on the red rays to a 

 greater distance than on the violet rays. There is no instance in 

 nature besides of a simple projectile moving with a velocity uniform in 

 all cases, whatever may be its cause, and it is extremely difficult to imagine 

 that so immense a force of repulsion can reside in all substances capable of 



