the Cause of coloured concentric Rings. 
2 99 
52. The critical Separation of the Colours , which takes place at 
certain Angles of Incidence , is the primary Cause of the New~ 
tonian coloured Rings between Object-glasses. 
It has been proved that streaks, concentric rings, lenticular 
figures, and all sorts of irregular coloured phenomena may 
be seen by means of the prism ; and in the 35th, gfith, and 
37th articles, it has already been sufficiently explained that the 
cause of the great variety of these appearances is to be found 
in the configurating power of surfaces. I have also remarked 
in the 40th article, that in order completely to account for the 
Newtonian rings, it remained only to be shown how the colours 
thus modified are produced. 
The prismatic experiments contained in this paper have ex- 
plained in what manner a critical separation of the colours, 
which takes place at certain angles of incidence, is the cause 
of the appearance of the blue and red bows ; since the diffe- 
rent reflexibility of the rays of light, by which Newton has 
accounted for the blue bow, brings on a critical separation of 
the blue colours, and since also the different intromissibility 
by which I have explained the red bow, occasions an equally 
critical separation of the red ones. 
In the 50th article I have not only proved that all the above 
described various appearances, which in the first part of this 
paper were produced by convex glasses, may be equally well 
obtained by the use of a prism, but have also shown that the 
great simplicity of this valuable optical instrument has cleared 
up great difficulties, by pointing out to us that the colours 
which are modified into such various shapes, are in all prisma- 
tic experiments exclusively produced by the critical separation 
