PHYSICAL SCIENCE. 



XXXI X 



luminous and dark, that were formed between the 

 two plates of glass in the preceding experiments, 

 and determined to be what they were by the 

 different thickness of air between the plates, and 

 having to that thickness the relation already 

 specified. A plate, of which the thickness was 

 equal to a certain quantity multiplied by an odd 

 number, gave always a circle of the one kind ; 

 but when the thickness was the same quantity 

 multiplied by an even number, the circle was of 

 another kind ; the light, in the first case, being 

 reflected; in the second, transmitted. 



Light penetrating a thin transparent plate, of 

 which the thickness was m, 3 m, 5 m, &c., was 

 decomposed and reflected ; the same light pene- 

 trating the same plate, but of the thickness 0, 

 2m, &m, &c., was transmitted, though in a cer- 

 tain degree also decomposed. The same light, 

 therefore, was transmitted or reflected, according 

 as the second surface of the plate of air, through 

 which it passed, was distant from the first, by the 

 intervals 0, 2 m, 4 m, or m, 3 m, 5 z ; so that it 

 became necessary to suppose the same ray to be 

 successively disposed to be transmitted, and to be 

 reflected at points of space separated from one 

 another by the same interval m. This constitutes 

 what Newton called Fits of easy transmission and 

 easy reflection. It constitutes one of the most 

 singular parts of his optical discoveries. It is a 

 necessary inference from the phenomena ; and 

 though the cause cannot be assigned, it must, 

 notwithstanding, be admitted as a general fact, 

 which enables us to explain various phenomena 

 that were unknown at the time when Newton 

 lived. 



Newton's explanation of refraction proceeds 

 on the theory that light is an emanation of par- 

 ticles moving in straight lines with incredible 

 velocity, and attracted by the particles of trans- 

 parent bodies. When, therefore, light falls 

 obliquely on the surface of such a body, its 

 motion may be resolved into two; one parallel to 

 that surface, and the other perpendicular to it. 

 Of these the first is not affected by the attraction 

 of the body, which is perpendicular to its own 

 surface ; and, therefore, it remains the same in 

 the refracted as it was in the incident ray. But 

 the velocity perpendicular to the surface is in- 

 creased by the attraction of the body ; and what- 

 ever be the quantity of that velocity, its square, 

 on entering the same transparent body, will 

 always be augmented by the same quantity. But 

 it is easy to demonstrate, that if there be two 

 right angled triangles, with a side in the one 

 equal to a side in the other, the hypothenuse of 

 the first being given, and the squares of their re- 

 maining sides differing by a given space, the 

 sines of the angles opposite to the equal sides 

 must have a given ratio to each other. This 

 'tmounts to the same thing with saying, that, in 



the case before us, the sine of the angle of inci- 

 dence is to' the sine of the angle of refraction in 

 a given ratio. This explanation of the law of 

 refraction is so satisfactory, that it affords a 

 strong argument in favour of the system which 

 considers light as an emanation of particles from 

 luminous bodies. 



Huygens, indeed, deduced from the hypo- 

 thesis of the vibrations of an elastic fluid, an ex- 

 planation of refraction which is highly ingeni- 

 ous, but not quite so satisfactory as the New- 

 tonian; though it be the fashion at present to 

 give a preference to the hypothesis of Huygens, 

 we think that very solid objections might be 

 started against it, were this the proper place for 

 such an attempt 



The square that is added to that of the per- 

 pendicular velocity of light, in consequence of 

 the attractive force of the transparent substance, 

 is properly the measure of the quantity of that 

 attraction, and is the same with the differences of 

 the squares of the velocities of the incident and 

 the refracted light. This is readily deduced 

 from the ratio of the angle of incidence to that 

 of refraction. When this is done for different 

 substances, it is found that the above measure of 

 the refracting power of different bodies is nearly 

 proportional to their densities, with the exception 

 of those which contain much combustible matter, 

 which is always accompanied by an increase of 

 refracting power. 



Thus the refracting power, (ascertained in the 

 way just mentioned) when divided by the density, 

 gives quotients not very different from one 

 another, till we come to combustible bodies, 

 when a great increase immediately takes place. 

 In air, for instance, the quotient is 5208, in rock 

 crystal 5450, in common glass nearly the same ; 

 but in spirit of wine, oil, and amber, the same 

 quotients are 1012 L, 12607, 13654. In the 

 diamond he found the quotient 14556. Hence he 

 conjectured that the diamond was, at least in 

 part, a combustible body. The refracting power 

 of water being great for its density, its quotient 

 being 7845, he concluded that an inflammable 

 substance enters into its composition a conjec- 

 ture which was verified nearly a century after- 

 wards by the synthetical experiments of Mr 

 Cavendish. 



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 con- 

 sequence 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 penetrate. In the case of 

 a transparent body, the light which, when it 

 arrives at this outwork is in &JU of easy reflec- 



