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of the Production of Colours. 
condition annexed to the explanation of the colours of thin 
plates, which involves another part of the same theory, that is, 
that where one of the portions of light has been reflected at the 
surface of a rarer medium, it must be supposed to be retarded 
one half of the appropriate interval, for instance, in the cen- 
tral black spot of a soap-bubble, where the actual lengths of 
the paths very nearly coincide, but the effect is the same as if 
one of the portions had been so retarded as to destroy the other. 
From considering the nature of this circumstance, I ventured to 
predict, that if the two reflections were of the same kind, made 
at the surfaces of a thin plate, of a density intermediate between 
the densities of the mediums containing it, the effect would be 
reversed, and the central spot, instead of black, would become 
white; and I have now the pleasure of stating, that I have fully 
verified this prediction, by interposing a drop of oil of sassafras 
between a prism of flint-glass and a lens of crown glass : the 
central spot seen by reflected light was white, and surrounded 
by a dark ring. It was however necessary to use some force, in 
order to produce a contact sufficiently intimate; and the white 
spot differed, even at last, in the same degree from perfect 
whiteness, as the black spot usually does from perfect blackness. 
The colours of mixed plates suggested to me an idea which 
appears to lead to an explanation of the dispersion of colours by 
refraction, more simple and satisfactory than that which I ad- 
vanced in the last Bakerian lecture. We may suppose that 
every refractive medium transmits the undulations constituting 
light in two separate portions, one passing through its ultimate 
particles, and the other through its pores ; and that these por- 
tions re-unite continually, after each successive separation, the 
one having preceded the other by a very minute but constant 
