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the Theory of Light and Colours. 21 
becomes necessary to suppose the number limited, for instance, 
to the three principal colours, red, yellow, and blue, of which 
the undulations are related in magnitude nearly as the numbers 
8, 7, and 6 ; and that each of the particles is capable of being 
put in motion less or more forcibly, by undulations differing 
less o,r more from a perfect unison ; for instance, the undula- 
tions of green light being nearly in the ratio of 6 ± , will affect 
equally the particles in unison with yellow and blue, and pro- 
duce the same effect as a light composed of those two species : 
and each sensitive filament of the nerve may consist of three 
portions, one for each principal colour. Allowing this statement, 
it appears that any attempt to producq a musical -effect from 
colours, must be unsuccessful, or at least that nothing more 
than a very simple melody could be imitated by them ; for the 
period, which in fact constitutes the harmony of any concord, 
being a multiple of the periods of the single undulations, would 
in this case be wholly without the limits of sympathy of the 
retina, and would lose its effect; in the same manner as the 
harmony of a third or a fourth is destroyed, by depressing it to 
the lowest notes of the audible scale. In hearing, there seems 
to be no permanent vibration of any part of the organ. 
HYPOTHESIS IV. 
All material Bodies have an Attraction for the ethereal Medium , 
by means of which it is accumulated within their Substance, and 
for a small Distance around them , in a State of greater Density, 
but not of greater Elasticity. 
It has been shewn, that the three former hypotheses, which 
may be called essential, are literally parts of the more compli- 
cated Newtonian system. This fourth hypothesis differs perhaps 
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