the Cause of coloured concentric Rings . 233 
the whole foundation on which the theory of the size of such 
parts is placed, will be taken away, and we shall consequently 
have to look out for a more firm basis on which a similar edi- 
fice may be placed. That there is such a one we cannot doubt, 
and what I have already said will lead us to look for it in the 
modifying power which the two surfaces, that have been 
proved to be essential to the formation of rings, exert upon 
the rays of light. The Second Part of this Paper, therefore, 
will enter into an examination of the various modifications 
that light receives in its approach to, entrance into, or passage 
by, differently disposed surfaces or bodies ; in order to dis- 
cover, if possible, which of them may be the immediate cause 
of the coloured rings that are formed between glasses. 
