Light, in consequence of these views, became, in the hands of Newton, the means of making important discoveries concerning the internal and chemical constitution of bodies. The square that is added to that of the perpendicular velocity of light in consequence of the attractive force of the transparent substance, is properly the • sure of the quantity of that attraction, and is the same with the difference of the squares of the velocities of the incident and the refracted light. This is readily deduced,.
therefore, from the ratio of the angle of incidence to that of refraction ; and when this is done for different substances, it is found, that the above measure of the refracting power of transparent bodies is nearly proportional to their density, with the exception of those which contain much inflammable matter in their composition, or sulphur, as it was then called, which is always accompanied with an increase of refracting power.' Thus, the refracting power, ascertained as above, when divided by the density, gives quotients not very different from one another, till we come to the inflam mable bodies, where a great increase immediately takes place. In air, for instance, the quotient is 5208, in rock-crystal 5450, and the same nearly in common glass But in spirit of wine, oil, amber, the same quotients are 10121, 12607, 13654. New ton found in the diamond, that this quotient is still greater than any of the preced ing, being Hence he conjectured, what has since been so fully verified by experiment, that the diamond, at least in part, is an inflammable body. Observing, also, that the refracting power of water is great for its density, the quotient, ex pounding it, as above, being 7845, he concluded, that an inflammable substance enters . into the composition of that fluid,—a conclUsion which has been confirmed by one of the most certain but most unexpected results of chemical analysis. The views thus suggested by Newton have been successfully pursued by future inquirers, and the action of bodies on light is now regarded as one of the means of examining into their internal constitution.
I should have before remarked, that the alternate disposition to be easily reflected and easily transmitted, serves to explain the fact, that-all transparent substances reflect a portion of the incident light. The reflection of light from the surfaces of opaque bodies, and from the 'anterior surfaces of transparent bodies, appears to be produced by a repulsive force exerted by those surfaces at a determinate but very small distance, in consequence of which there is stretched out over them an elastic web through which the particles of light, notwithstanding their incredible velocity, are not always able to In the case of a transparent body, the light which, when it arrives at this outwork, as it may be called, is in a fit of easy reflection, obeys of course the repulsive force, and is reflected back again. The particles, on the other hand, which are in the state which disposes them to be transmitted, overcome the repulsive force, and, entering into the interior of the transparent body, are subjected to the action of its at tractive force, and obey the law of refraction already explained. If these rays, how
ever, reach the second surface of the transparent body (that body being supposed denser than the medium surrounding it), in a direction having a certain obliquity to that surface, the attraction will not suffer the rays to emerge into the rarer medium, but will force them to return back into the transparent body. Thus the reflection of light at the second surface of a transparent body is produced, not by the repulsion of the medium in which it was about to enter, but by the attraction of that which it was pre paring to leave.
The first account of the experiments from which all these conclusions were de duced, was given in the Philosophical Transactions for 1672, and the admiration excited by their brilliancy and their novelty may easily be imagined. Among the men of science, the most enlightened were the most enthusiastic in their praise. Huy gens, writing to one of his friends, says of them, and of the truths they were the means of making known, " Quorum respectu omnia huc usque edita jOunia sent et prorsus puerilia." Such were the sentiments of the person who, of all men living, was • the best able to judge and had the best right to be fastidious in what related to optical experiments and discoveries. But all were not equally candid with the Dutch phi losopher ; and though the discovery now communicated had every thing to recom mend it which can arise from what is great, new, and singular; though it was not a theory or a system of opinions, but the generalization of facts made known by experi ments; and though it was brought forward in the most simple and unpretending form, a host of enemies appeared, each eager to obtain the unfortunate pre-eminence of being the first to attack conclusions which the unanimous voice of posterity was to confirm. In this contention, the envy and activity of Hook did not fail to give him the advantage, and he communicated his objections to Newton's conclusions concern ing the refrangibility of light in less than a month after they bad been read in the Royal Society. He admitted the accuracy of the experiments themselves, but de nied that the cause of the colour is any quality residing permanently in the rays of light, any more than that the sounds emitted from the pipes of an organ exist originally in the air. An imaginary analogy between sound and light seems to have been the basis of all his optical theories. He conceived that colour is no thing but the disturbance of light by pulses propagated through it ; that blackness proceeds from the scarcity, whiteness from the plenty, of undisturbed light ; and that the prism acts by exciting different pulses in this fluid, which pulses give rise to the sensations of colour. This obscure and unintelligible theory (if we may honour what is unintelligible with the name of a theory) he accompanied with a multi tude of captious objections to the reasonings of Newton, whom he was -not ashamed to charge with borrowing from him without acknowledgment: To all this Newton replied, with the solidity, calmness, and modesty, which became the understanding and temper of a true philosopher.