Error of the coittliion BoBrliie of the var'iotis RefcxilUlty of Light. S95 



^inciples which have been laid down, the colours of natural bodies may be explained. The 

 celebrated difcovery of Newton, that thefe depend on the thicknefs of their parts, is de- 

 graded by a compnrifon with his hypothefis of the fits of rays and waves of ether. De- 

 lighted and aftonidied by the former, we gljdly turn from the latter ; and unwilling to iil- 

 volve in the fmoke of unintelligible theory fo fair a fabric, founded on (Irid induaion, we 

 with to find fome continuation of experiments and obfervations, which may relieve us froni 

 the neceffity of the fuppofition. My fpcculations on this fubjed have by no means been 

 completed, as I have not yet finiflied the demonftrations and experiments into which it has 

 engaged me to enter ; but in order to complete my plan 1 fliall offer a few hints on the fub- 

 jea : -The parts of light are affirmed in Prop. III. Book 1. Part I. of the Optics to be dif- 

 ferent in reflexibility ; that is, according to the author's definition, in difpofuion to be turned 

 back, and not tranfmitted at the confines of two tranfparent media. That the demonftra- 

 tion involves a logical error, appears pretty evident. When the rays, by refraftion through 

 the bafe of the prifm ufed in the experiment, are feparated into their parts, thefe become 

 divergent, the violet and red emerging at very different angles, and thefe were alfo incident 

 on the bafe, at different angles from the refraftion of the fide at which they entered : when 

 therefore the prifm is moved round on its axis, as defcribed in the propofition, the bafe is 

 neareft the violet, from the pofition of the rays by refradion, and meets it firft ; fo that 

 the violet being refleded as foon as it meets the bafe, it is refieiSled before any of the other 

 rays, not from a different difpofition to be fo, but merely from its different refrangibility. 

 Although then this experiment is a complete proof of the difl^erent refrangibility of the rays, 

 it proves nothing elfe ; and indeed an experiment will convince us that the rays all have 

 the fame difpofition to be reflected, provided the angle of incidence be the fame. For I held 

 a prifm vertically, and let the fpeftrum of another prifm be reflected by the bafe of the 

 former, fo that the rays had all the fame angle of incidence ; then turning round the vertical 

 prifm on its axis when one fort of rays was tranfmitted or reflefted, all were tranfmitted or 

 renewed. We cannot therefore apply the different reflexibility of light to the explanation 

 of the colours of bodies, fince this property has no exiftence. But we have fhewn that 

 the rays differ in reflexibility, taking the word in the new fenfe, as explained above. Let 

 us fee whether this principle will not folve the important problem. It is evident that the 

 particles of bodies are fpecular. Now, I take the colours of bodies to depend not on the 

 fize, but on the pofition, of thefe particles, or at lead on only tlie fize in as far as it in- 

 fluences their pofition ; an idea perfedly familiar to mathematicians. 



Obf. 16. In making fome of the experiments which I related above, on the reflexibility 

 of light, I obferved, among the regular images made by moft of the pins which I ufed, one 

 or two all of the fame colour, as red, blue, &c. ; and when the pin was moved thete moved 

 alfo, becoming of other colours in regular order like the red; ; which (hows plainly that 

 their being of one colour at firft was owing to fome fibre in the furface jutting out, or ra- 

 ther to feveral of thefe, which ftopped the red and all the reft but the blue of fcveral images, 

 or the blue and all the reft but the red. Farther, I produced fevcral regular images by two 

 or three very fmall pins, and with confiderable trouble. I at laft contrived to place them 

 in fuch a pofition, as that one blue colour of confiderable fi/e might be produced, then a 

 red, and fo on, by altering the pofturc of the pins. Now whether the pofture or the fize be al- 

 tered it matters not, for the one aflre(51s the otiier. Is it not evident that this experiment, 



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