394< Dr. Young's Account of some Cases 
interval, depending on the regular arrangement of the particles 
of a homogeneous medium. Now, if these two portions were 
always equal, each point of the undulations resulting from their 
re-union, would always be found half way between the places 
of the corresponding point in the separate portions ; but, sup- 
posing 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 mag- 
nitude of those undulations ; so that the larger undulations will 
be somewhat further advanced after each re-union than the 
smaller ones, and, the same operation recurring at every par- 
ticle of the medium, the whole progress of the larger undula- 
tions will be more rapid than that of the smaller ; 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 sepa- 
ration of colours by thin plates. I have repeated his experiments 
on the spectrum with perfect success, and have made some 
attempts to procure comparative measures from thin plates; 
and I have found that, as Sir Isaac Newton has already 
