An Account of Some Cases of the Production ofColours, Not Hitherto Described. Thomas Young.

                                the preceding portion to be the smaller, the newly combined 
undulation will be less advanced than if both had been 
equal, and the difference of its place will depend, not only 
on the difference of the length of the two routes, which 
will be constant for all the undulations, but also on the 
law and magnitude of those undulations, <s>the larger advancing <\s> so that the larger undulations will be some-
what further advanced after each reunion than the smaller 
ones; and the same operation recurring at every particle of 
the medium, the whole progress of the larger undulations will 
be more rapid than that of the smaller; and hence the 
deviation, in consequence of the retardation of the motion of 
light in a denser medium, will of course be greater for the 
smaller than for the larger undulations. Assuming the law 
of the harmonic curve for the motions of the particles, we might 
without much difficulty reduce this conjecture to a comparison 
with experiment, but it would be necessary, in order to warrant 
our conclusions, to be provided with very accurate measures 
of the refractive and dispersive powers of various substances, for 
rays of all descriptions.
Dr. Wollaston’s very interesting observations 
would furnish great assistance in this inquiry, when compared 
with the separation of colours by thin plates. I have repeated 
his experiment on the spectrum with perfect success, and 
I have made some attempts to procure comparative measures 
from thin plates; and I have found, that, as Sir Isaac Newton 
has already observed, the blue<s>s<\s> and violet<s>s<\s> light is more 
dispersed by refraction than in proportion to the difference of