2G4 Dr Herschei/s Experiments for investigating 
applied to each other, as it will hereafter be shown that when 
the incumbent plain glass is not of a parallel thickness, co- 
loured phenomena may be rendered visible between two per- 
fectly plain surfaces, although no force or strain should be 
used to produce a fallacious, curved, contact. 
40. Of the Production of coloured Appearances. 
Hitherto I have only considered the coloured rings which 
Sir Isaac Newton has pointed out, and have shown, at the 
end of the 28th article, that no more than two surfaces are 
essential to their formation. It has now also been proved, that 
the configuration of the coloured phenomena arises from the 
curvature of one or both of the two essential surfaces. From 
these principles it will be seen, that we are to distinguish be- 
tween the production of the colours and that of their configu- 
ration when produced. By the experiments that have been 
given, the cause of the configuration is laid open to our view; 
but the production and arrangement of the colours remain to 
be investigated. 
The leading feature of the arrangement of the colours of 
the rings is prismatic ; that is to say their order is red, orange, 
yellow, green, blue, indigo, and violet ; in order, therefore, 
to enter minutely into the subject, I shall have recourse to 
some prismatic experiments. 
It will be necessary here to mention, that the proposed enu- 
meration of the modifications of light, which was intended to 
have been given in this part of my paper, is grown to such 
an extent by the number of experiments I have made upon 
the subject, that its introduction would occasion a long inter- 
ruption of the present subject ; and although undoubtedly the 
