132 ON COXOUES OF THIN PELLICLES. 



to each molecule of a body, in a case given to deduce the 

 motion of the luminous rays repelled or impelled now in one 

 direction, now in another, conformably to the reflections or 

 transmissions produced by pelHcles. 

 The same co- It is of more importance to my object however to reraaik, 

 lours ; roduced ^i^,^^ ^| i ..g .^y\^\^^^ from fits of eusy reflection and trans- 

 b^'twe, II bodies _ ° /^ "^ , 



in a vacuum, mission are equally produced, as Mazeas very justly ob- 

 served, betAvcen the surfaces of two bodies brought near to- 

 gether, witliout the interposition of any matter, as in two 

 lenses, or two pieces of glass, applied to each other in the 

 vacuum of an airpump. 

 And in thick On the other hand these colours do not always require a 

 P ^-'^s. ^^^^,y. ^^^^Y distance of the surfaces, since Newton himself 



obtained coloured rings by the action of two surfaces of a 

 concave glass mirror three lines thick; and found, that in 

 thick plates these rings depend on the ratio of the thickness, 

 according to the same law as he had determined with respect 

 to thin plates; which he confirmed by the observation of the 

 rings of a mirrOr only one line thick. 

 Colours of pel- ^^^ ^^^ *^^^" ^-*y comparing the various phenomena I have 

 licles indepen- mentioned, that the prismatic colours of a pellicle, or a thin 

 dautof ; icsu - pj^^^^ ^f glass, are as fugitive and independant of the proper 

 colour of the substance, as those of a thick piece of glass : 

 and of its thin- that those colours even may not depend on the thickness in 

 uess. jjjjy respect, as when they arise in the interval between two 



glasses brought together, or in the fissures of certain mine- 

 Analogous to ^'^^s • *^^^* ^^^^y ^^^'^ ^^^ greatest analogy with the rings pro- 

 the rings iava- duced in a mist, in smoke, or in the intervals of threads 

 tween'onake impermeable themselves to liglit: and finally, that, if we 

 substances. trace it up to the action of a point, or a single particle of 

 And m?y be matter, on the luminou? fluid, we shall there find a very pro- 

 aTtton of par- bable origin of the modifications of the direction of the 

 tides of matter rays, that are deflected by the particles of bodies in the dif- 

 °" 'g t, ferent instances quoted, and which, being -difl^erently influ- 



enced each according to its nature, ultimately escape in a 

 diflerelit direction. Hence results a variety of colours on 

 without any these bodies, determined solely by the number or distance 

 the'ir n^tu're ^^ ^^^'^ particles, without any relation to their nature. 

 Colours of bo- Let US now proceed to establish^a parallel between these 



• sorts 



