750 PHILOSOPHICAL TRANSACTIONS. [aNNO I/QS. 



fire-marble of Saxony, &c. All these bodies having surfaces of a fibrous struc- 

 ture, each fibre reflects and decompounds the rays. 



4. The consideration of the foregoing phenomena inclined me to think, that 

 on the principles which have been laid down, the colours of natural bodies may 

 be explained. The celebrated discovery of Newton, that these depend on the 

 thickness of their parts, is degraded by a comparison with his hypothesis of the 

 fits of rays and waves of ether. Delighted and astonished by the former, we 

 gladly turn from the latter ; and unwilling to involve in the smoke of unintelli- 

 gible theory so fair a fabric, founded on strict induction, we wish to find some 

 continuation of experiments and observations which may relieve us from the 

 necessity of the supposition. My speculations on this subject have by no means 

 been completed, as I have not yet finished the demonstrations and experiments 

 into which it has engaged me to enter ; but, in order to complete my plan, I 

 shall offer a few hints on the subject. The parts of light are affirmed, in prop. 

 3, book 1, part 1, of the Optics, to be different in reflexibility ; that is, according 

 to the author's definition, in disposition to be turned back, and not transmitted 

 at the confines of two transparent media. That the demonstration involves a 

 logical error appears pretty evident. When the rays, by refraction through the 

 base of the prism used in the experiment, are separated into their parts, these 

 become divergent, the violet and red emerging at very different angles, and these 

 were also incident on the base at different angles, from the refraction of the side 

 at which they entered ; when, therefore, the prism is moved round on its axis, 

 as described in the proposition, the base is nearest the violet, from the position 

 of the rays by refraction, and meets it fii'st ; so that the violet being reflected as 

 soon as it meets the base, it is reflected before any of the other rays, not from a 

 difterent disposition to be so, but merely from its different relrangibility ; though 

 then this experiment is a complete proof of the different refrangibility of the 

 rays, it proves nothing else ; and indeed an experiment will convince us, that 

 the rays all have the same disposition to be reflected, provided the angle of inci- 

 dence be the same. For I held a prism vertically, and let the spectrum of ano- 

 ther prism be reflected by the base of the former, so that the rays had all the 

 same angle of incidence ; then turning round the vertical prism on its axis, when 

 one sort of rays was transmitted or reflected, all were transmitted or reflected. 

 We cannot therefore apply the different reflexibility of light, to the explanation 

 of the colours of bodies, since tliis property has no existence. But we have 

 shown that the rays dift'er in reflexibility, taking the word in the new sense, as 

 explained above; let us see whether this principle will not solve the important 

 problem. It is evident that the particles of bodies are specular. Now I take 

 the colours of bodies to depend, not on the size, but on the position of these 

 particles, or at least on only the size in as far as it influences their position ; an 

 idea perfectly familiar to mathematicians. 



Obs, l(). In making some of the experiments, which I related above on the 



