of equal thicknefs, provided they have the fame 
denfity. He concludes this whole dodtrine in thefe 
words : I have hitherto explained the powers of 
bodies to refledl and refradt, and (hewed that 
“ thin tranfparent plates, fibres, and particles, do, 
according to their feveral thickneffes and denfitiesy 
“ refledt feveral forts of rays, and thereby appear 
“ of feveral colours \ and by confequence, that no- 
thing more is requifite, for producing all the 
“ colours of natural bodies, than the feveral fisc,es 
“ and denfities of their tranfparent particles.” 
Though he has accurately fhewn what colours 
arife from the feveral changes of thicknefs, I do not 
find that any one has attempted to explain in what 
7na?mer the differences of denfty, in the component 
particles of bodies, contribute to produce the feve- 
ral differences of colours: and therefore I thought, 
that if inftances could be produced of bodies whofe 
feveral differences of colour appear to be propor- 
tioned to their feveral degrees of de77fity, it would 
tend to illuftrate this part of Optics. 
To this purpofe, however, are conducive all thofe 
experiments and obfervations, from which Sir 
Kaac Newton has inferred that bodies have their 
refradiive and refleBive powers nearly praportional 
to their denfities ; and that the leaf refrangible rays 
require the greatef power to refledt them : which 
is deducible froni/ hence, i. that the rays are 
refledted at the greatef obliquity of incidence, and 
the nsiolet at the leaf, 2 . that the niolet is refiedted, 
in like circurnftances, at the leaf thicknefs of any 
thin plate or bubble, the red at iht greatef thick- 
nefs, and the intermediate colours at intermediate 
C 2 thickneffes. 
