An extract of Mr. Isaac Newton's letter, written to the published from Cambridge April 3.1673. concerning the number of colors, and the necessity of mixing them all for the production of white ; as also touching the cause why a picture cast by glasses into a darkned room appears so distinct notwithstanding its irregular refraction: (which letter, being an immediat answer to that from Paris, printed no. 96. p. 6086. of these tracts, should also, if it had not been mis-laid, have immediately followed the