OF NEWTON'S OPTICS. 



55 



which is perfectly transparent, will fre- 

 quently, when mixed, be strongly co- 

 loured. This may be conceived to arise 

 from the mutual action of the corpus- 

 cles of the two liquids : they either create 

 or destroy the connexion of particles, 

 and cause the molecules to swell or 

 shrink, whereby not only their bulk but 

 their density may be changed. 



This change of the magnitudes of the 

 particles and their interstices is an ade- 

 quate cause for the change of colour, 

 according to Newton's theory. 



It is observable in experiments such 

 as those described in the last chapter, 

 that when the thin transparent medium 

 which reflects the colours is more dense 

 than the surrounding medium, the co- 

 lours are much more vivid and brisk, 

 and less liable to shift their tints with a 

 change in the direction in which they 

 are viewed. This will be evident, by 

 comparing the effect of thin films of 

 glass or mica, the ambient medium 

 being air, with that of a thin plate of air, 

 inclosed between two pieces of plate 

 glass. From this circumstance, Newton 

 supposes that the parts of bodies on 

 which their colours depend must be 

 denser than those which fill the remain- 

 ing spaces within the surface of the 

 body. For the colour of a body being 

 generally produced by light reflected at 

 all angles, if the particles reflecting 

 colour were rarer than the surrounding 

 medium, all tints would be reflected at 

 different obliquities, and so the body 

 would appear white or grey. But if the 

 parts reflecting colour be more dense 

 than the others, the lights reflected nearly 

 perpendicular will, by predominating 

 over the oblique ones, give their own 

 colour to the body. 



(77.) All the preceding reasoning led 

 Newton directly to the inference, that the 

 colour of a body furnishes a means of 

 determining the magnitude of the ulti- 

 mate transparent corpuscles of which it 

 is composed. Many circumstances render 

 it probable, as Newton conceived, that 

 the parts of bodies have, for the most 

 part, the same refractive density as those 

 of water or glass. This being assumed, 

 it follows that the diameter of the cor- 

 puscle of a body, which has any pro- 

 posed colour, is equal to the thickness 

 of a plate of water or glass, which would 

 reflect the same colour, and which may 

 always be determined by the Table, p. 52, 

 and by the Scale, fig. 46. Thus, if it be 

 desired to know the diameter of a cor- 

 puscle, which, being of equal density 



with glass, will reflect green of the third 

 order; 16J expresses the number of 

 millionth parts of an inch in it. 



(78.) The received opinion respecting 

 the cause of reflection of light was, that 

 its particles impinged upon the hard sur- 

 face of the solid parts of the reflecting 

 body, and were reflected by the reaction 

 of that surface, in the same manner as 

 an elastic sphere is reflected when it 

 strikes a hard plane. This opinion 

 seemed to be countenanced by the law 

 of reflection of light, which is the same 

 as that which regulates the reflection of 

 all elastic spheres impinging upon hard 

 surfaces. Newton, however, held that 

 light does not impinge on the solid and 

 impervious parts of bodies, but is re- 

 flected without having encountered these 

 surfaces. He shows many difficulties 

 which attend the hypothesis, that light 

 impinges on the solid parts of bodies in 

 reflection, among which the following 

 may be noticed. 



If light, after passing through glass, 

 be reflected by the surface, the ambient 

 medium being air, the reflection will be 

 stronger than it would be if the light 

 had passed through the air and been 

 reflected by the surface of the glass, 

 and much stronger than if the ambient 

 medium had been water. This effect 

 would lead to the conclusion, that the 

 particles of air repelled the light with 

 greater force than those of either water 

 or glass. But it is still more unac- 

 countable on this theory, that upon 

 withdrawing the air from about the 

 glass by an air-pump, the reflection of 

 the light from the surface which sepa- 

 rates the glass from the vacuum is still 

 stronger than in either of the former 

 cases. 



If the light, after passing through the 

 glass, be incident on the surface more 

 obliquely than at an angle of 41, it will 

 be wholly reflected ; but, at all obliqui- 

 ties less than this, it will be transmitted. 

 Hence, if we admit that the impact on 

 solid parts is the cause of reflection, 

 and therefore the penetration of in- 

 terstices the cause of transmission, we 

 are compelled to suppose, that at obli- 

 quities greater than 41, all the parts of 

 the light encounter solid parts of air, 

 but at less obliquities they all pass into 

 pores or interstices ; and, on the other 

 hand, that in passing through air, and 

 impinging upon glass, it never fails to 

 meet pores enough to transmit nearly 

 the whole of it, even at the most oblique 

 incidences, Some may suppose that 



