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III. A Letter to the Right Honourable the 
Earl of Morton, Prfident of the Royal 
Society, Containing Experiments and Ob- 
fervations on the Agreement between the 
Specific Gravities of the fever al Metals^ and 
their Colours when united to Glafs^ as well 
as thofe of their other Prppad^ions: By 
Edward Delaval, F. R, S. M. A, and 
Fellow of Pembroke Hall, Cambridge. 
My Lord, 
Read Jan. 24, T" T A K E the liberty of laying before 
>765* Jl^ your Lordlhip the following paper, 
containing a variety of fad:s and experiments, which 
I have endeavoured to apply to fome Optical in- 
quiries. 
Befides the experiments originally contrived and 
made by myfelf, I have repeated moll of thofe 
which I have quoted from others. 
Sir Ifaac Newton, in his Optics, has fliewn hy 
a feries of experiments, that the feveral differences 
of colours, exhibited by thin tranfparent plates, are 
occafioned by their feveral thickneffes; and that 
therefore the tranfparent parts of bodies do, ac- 
cording to their different fizes, refledt rays of one 
colour and tranfrnit thofe of another; and confe- 
quently that the bignefs of the component parti- 
cles of natural bodies may be conjedured from 
their colours ; fince the particles of thofe bodies 
moft probably exhibit the fame colours as a plate 
4 of 
