( 6°6 ) 
eight Foot from r r, I fixed the Lens L L, of four 
Foot Focus, in a Frame upon another Table, with its 
Axis going through the Middle of rrv At the Diftance 
of about eight Foot beyond the Lens, 1 fet up the 
Skreen or Square of white Paper S. Having put into 
the Hole rra ftiff Paper, painted with Vermillion, 
and wrapped four Times and an half with black Silk 
(as reprefented by R), that Paper enlighten’d by the 
Candles at /, k , the Image of the red Paper was pro- 
jected upon the Skreen at p, and when the mod diftind 
Place was found, the Skreen was fixed : Then a Paper 
painted with Ultramarine being fixed in the Hole r r, 
the Image of it was fo indiftind at p, that the Images 
of the black Silks could not be feen^ but holding a 
Piece of Paper clofe to the Skreen, and bringing it 
forward, at about { of an Inch from the Skreen, the Re- 
prefentation of the Silks began to appear on the blue 
Image ; but it was moft diftind at an Inch and {, or at 
Z Z j fo that there was U Inch between the diftind 
Bafe of the red \ and that of the blue Paper. But 
what has led feveral People into an Error in making 
this nice Experiment, is the Depth of the Focus of the 
Rays in both Cafes ; for though the red Image was mod 
diftind at p, yet the Reprefentation of the black Silks 
might juft be perceived by a good Eye when the Skreen 
was moved backwards or forwards i of an Inch : The 
blue Image Which was ftronger had its Silks vifible an 
Inch on either Side of Z Z ; fo that in a Paper half 
red and half blue, painted with thefe Colours, one 
might have feen the Silks (though faintly) upon the 
two Images at once, and have been thereby deceived : 
But | of an Inch beyond the Place common to both^ 
the red alone would have appeared diftind ; and an 
Inch 
