OF NEWTON'S OPTICS, 



53 



principles to which bodies owe this apti- 

 tude to reflect some lights rather than 

 others, and to trace this aptitude to the 

 internal constitution and essential nature 

 of the bodies. 



A very close analogy may be ob- 

 served between the phenomena of re- 

 flection and refraction. It has been 

 proved that those species of light which 

 were most refrangible were also most 

 reflexible. But it further appears that 

 those svirfaces at which light is most 

 powerfully refracted have a propor- 

 tionately intense power of reflection. 



A surface which separates glass from 

 air has a considerable refractive power. 

 That its reflective power is also great, 

 will be seen by looking into the side of 

 a prism at its base, the eye being placed 

 so as to view the base obliquely and the 

 side perpendicularly. The reflection of 

 light from the base will be so intense, 

 that every object seen in it has all the 

 vivid splendour of reality. 



The surface which separates glass 

 from water has a much less refracting 

 power, and, accordingly, we find the 

 reflective power proportionately dimi- 

 nished. If the prism just mentioned be 

 laid with its base upon water, the splen- 

 dour of the reflection will be considerably 

 impaired, and the images seen in the 

 base will no longer have the intense 

 brilliancy which was observable when 

 the medium below the base was air. To 

 this cause the diamond owes its lustre. 

 Its refracting power is greater than that 

 of most other substances. In confor- 

 mity with the principle here laid down, 

 its reflecting power is proportionately 

 great, and we find the reflection of light 

 from its inner surfaces of a correspond- 

 ing intensity. On the other hand, if a 

 surface separates two media of equal 

 refracting powers, no refraction takes 

 place in passing from the one medium to 

 the other, and so neither do we find any 

 reflection. Thus, at the point at which 

 the lenses touched in the experiment 

 described in (54), no light was reflected 

 at whatever obliquity it was viewed. 



(72.) Newton, therefore, concludes that 

 the reason why uniform pellucid sub- 

 stances, such as water, glass, or crystal, 

 reflect no light except from their surfaces, 

 is, because every part within those sur- 

 faces has an uniform refracting power, or 

 has in every place the same density. He 

 infers that reflection of light may always 

 betaken as an indication of a change 

 of density in the medium at that point 

 where the reflection is made, and that it 



is a test by which a change of density 

 may be as certainly ascertained as by 

 refraction. 



A ray of 'light falling on the surface 

 of a body in a fit of easy transmission, 

 enters it. 'if, after penetrating to a small 

 depth, it meet with a point at which 

 there is a change of density, and be at 

 the same time in a fit of easy reflection, 

 it will retrace its course. According to 

 this view, every body, how opaque so 

 ever it be, is transparent to a certain 

 depth within its surface. Experiments 

 sufficiently evince the truth of this pro- 

 position. If a very thin lamina of the 

 most opaque substance be suspended 

 before a hole through which a beam of 

 light is admitted into a dark room, it 

 will be manifestly transparent, and light 

 will be perceptible through it, provided 

 it be sufficiently attenuated. 



From all this it seems not impossible 

 that the intimate particles or molecules 

 of opaque or coloured bodies, are sepa- 

 rated by minute pores or spaces, which 

 are either entirely void or filled with 

 some subtle material of a different den- 

 sity from the molecules of the body, in 

 the same manner as a liquid pervades the 

 particles of a solid substance which it 

 holds in solution. If we admit that light 

 penetrates the external surface of opaque 

 bodies, a fact which experience proves,we 

 must also admit the existence of spaces 

 within that body, filled by some medium 

 differing in density from the molecules 

 of the body itself ; for, without this dif- 

 ference of density, there could be no re- 

 flection, and, consequently, no opacity. 



(73.) There are many experiments by 

 which phenomena are elicited, which 

 seem to support the hypothesis that the 

 discontinuity of parts is the cause of opa- 

 city. If the pores of an opaque body be 

 filled with any substance of nearly the 

 same density or refracting power, it loses 

 its opacity, and becomes diaphanous. 

 Paper and oil have nearly equal refract- 

 ing powers. Dry paper is opaque ; but, 

 when steeped in oil until its pores are in 

 a great degree filled with that fluid, it 

 acquires a proportionate degree of trans- 

 parency. In the same manner, linen 

 cloth, and many other substances of 

 very imperfect transparency, will have 

 that quality perceptibly increased by 

 being steeped in liquids whose refracting 

 power is nearly equal to their own. 



On the other hand, the most trans- 

 lucent bodies may be rendered opaque 

 by extricating from their pores the 

 matter which pervades them, which may 



