60 



OPTICS. 



much more light than any of the other 

 bodies which follow them in the table ; 

 and water, ice, and tabasheer, at the 

 foot of the table, reflect much less 

 light than any of the bodies which 

 precede them. In like manner, if we 

 pour castor oil upon crown glass, 

 which have nearly the same refractive 

 power, there is almost no light reflected 

 from their separating surface. If we 

 pour sulphuric acid on the same glass, 

 the reflective power of the surface is in- 

 creased. With alcohol it is still farther 

 increased ; with tvater, still farther ; and 

 when the glass is placed in air, the re- 

 flective power is a maximum. 



2. The smallest parts of almost all 

 natural bodies are, in some degree, trans- 

 parent, and the opacity, or impervious- 

 ness to light, of these bodies, arises from 

 the multitude of reflexions produced in 

 their internal parts. 



Gold and silver leaf are both transpa- 

 rent ; and as metallic salts, and the solu- 

 tions of all metals are perfectly transpa- 

 rent, we may regard the proposition as 

 established for the most opaque of all 

 substances. The blackest and most 

 opaque of stones, &c. become translu- 

 cent, and even transparent, uhen a 

 strong light is transmitted through the 

 sharp edges of small fragments of them. 



3. Between the parts of opaque and 

 coloured bodies there are many spaces, 

 which are either empty or are filled 

 with media of different densities; as 

 water ,for example, between the particles 

 with which any liquor is coloured; 

 air between the aqueous globules that 

 constitute clouds and mists ; and for the 

 most part space without either water 

 or air, but yet perhaps not wholly with- 

 out any substance, between the parts of 

 hard bodies. 



The truth of this proposition may 

 be deduced from the two preceding 

 ones ; for, by Prop. 2, there are many 

 reflexions made by the internal parts 

 of bodies, which, by Prop. 1, would 

 not happen if the parts of those 

 bodies were continuous without any 

 interstices between them, and of the 

 same refractive power. Besides, many 

 Iransparent bodies, such as minerals 

 and salts, become opaque when their 

 water of crystallization is driven off by 

 heat ; and many opaque bodies become 

 transparent by filling their pores with 

 water or oil. Hydrophane and opaque 

 tabasheer are perfectly transparent ; the 

 former when it has absorbed water, and 

 the latter when it has absorbed oil. 



Paper, vellum, and linen become trans- 

 parent in oil ; and iodine, a dark, me- 

 tallic, and opaque substance, when 

 driven off in vapour by heat, forms a 

 transparent, purple coloured gas. 



4. The parts of bodies and their in- 

 terstices must not be less than of some 

 definite size, to render them opaque and 

 coloured. 



The experiments in Chap, xiii., on 

 thin plates, completely prove, that be- 

 low a certain degree of thickness bodies 

 have no power to reflect light ; that is, 

 are black ; and this is finely illustrated 

 by the black down of quartz mentioned 

 in the same chapter. Hence it is clear, 

 that if the particles of all terrestrial 

 bodies were so small as, or smaller 

 than, the eight millionthpart of an inch, 

 (TOOOOO>, every object in the animate 

 and inanimate world would be absolutely 

 black, and consequently invisible ; for the 

 sun, planets, and stars could only show 

 us their own individual positions in the 

 sable firmament. The transparency of 

 water, glass, &c., Sir Isaac Newton 

 conceives to arise from this that 

 though they are as full of pores, or in- 

 terstices, between their parts as other 

 bodies are, yet their parts and interstices 

 are too small to cause reflexion at their 

 common surfaces. 



5. The transparent parts of bodies, 

 according to their several sizes, reflect 

 rays of one colour and transmit those of 

 another, for the same reasons that thin 

 plates, or minute particles of air, water , 

 and glass, reflect or transmit those 

 rays ; and this is the cause of all their 

 colours. 



If a body, such as a film of mica, 

 appears all over of one uniform colour, 

 blue, for example ; then, if it is cut into 

 threads, or broken into fragments, these 

 portions will still be blue ; and, conse- 

 quently, a heap of these blue portions 

 will constitute a mass, or powder, of a 

 blue colour. And as the parts of all 

 natural bodies are like so many frag- 

 ments of a thin plate, they must, for the 

 same reasons, exhibit the same colours. 



This conclusion appears also, by 

 examining the similarity between the 

 colours of natural bodies and those of 

 thin plates. The finely coloured feathers 

 of the humming birds, and those of 

 peacocks' tails, appear in the very same 

 part of the feather of different colours 

 in different positions of the eye ; the 

 colour descending in the scale as they 

 are seen more obliquely, as is the case 

 with the colours of thin plates. Hence 



