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made up of Orange and Yellow ; b 6 B a Ray made up of 
Green and Blue or a Sea Green, and b P a purple Ray. 
If from the place £ e you look full upon the Point b, the 
Spectrum or Image of the Candle at b will appear double, 
but not mix’d ; that is, there will appear a Sea-green Spot 
and a red Spot, as it were, one upon another; but notfo 
as to produce a mix’d or intermediate Colour. Then if 
the right Eye or Eye at £ be fhut, there will appear on- 
ly a green Spot to the Eye at e ; if the Eye at e be ihut, 
the Eye at £ will lee only a red Spot. 
If you come nearer to b t fo that the Eyes at e i, e i 
receive the moil and the leaft refrangible Rays, there will 
be a double Spettrum, viz. a red and a purple one juft 
touching, or upon one another : and the Phenomenon 
will anfwer as before. (Fig. z$ ). 
If keeping both Eyes open, you direcft their Axes to- 
wards 0 a Point nearer than the ufual place of the com- 
pound SpeftrttmS > (Fig.z6.) which Point is in a Line from 
the Nofe Nto the Point S ; or in other Words, if you 
look full at 0, or at the End of your Finger held inO, the 
red and the blue (or purple Spot) will appear to be divided 
from each other after the manner reprefented at p r (in 
Tig. 2 7 ) where the Red will appear to be on the Right- 
hand, and the Blue on the Left. 
To make plain what is meant by feeing the Spettra p and 
r whilft we look full at 0, T beg leave to explain the Di- 
ftin&ion between looking and feeing ; that I may the bet- 
ter Ihew how this Phenomenon proves that the Senfati- 
on of different Colours is caus’d by Rays differently re- 
fradted. 
I. Definition . 
The Optic Axis is a Line which going thro* the Center 
©f the Convexity of all the Coats and Humours of the 
A a a a Eye 
