the Cause of coloured concentric Rings. 275 
spherical surface, such as an object glass, under the prism it 
will immediately give us several sets of innumerable concen- 
tric coloured rings ; and, as will now be readily expected, a 
cylindrical surface placed under the prism will give a number 
of lenticular appearances, such as are contained between the 
intersections of two circular arches drawn concave towards 
each other. The irregular surface of mica will in like manner 
produce multiplications of appearances, that may be seen much 
better than they can be described. 
When the same surfaces are applied to the red bow, phe- 
nomena that are perfectly of the same form will be made 
visible within and just under the bow ; and the streaks will 
also be in a parallel direction. 
The side of the prism, to which a plain glass must be ap- 
plied, is of singular use in the explanation of many appear- 
ances of the coloured phenomena, which are to be seen, and 
it is on this account that the formation of the generated colours 
into all sorts of configurations has been noticed before I come 
to that part of this paper, wherein this subject must find a 
further discussion ; for by the application of a slip of plain 
glass, we can decisively ascertain the nature of any coloured 
appearance in the prism. Thus, when we see a common 
coloured red or blue arch, occasioned by the mere different 
refrangibility of light, the plain glass any how applied to the 
prism will give no streaks. If we apply the plain glass to a 
transmitting side, we can have no streaks from a critical blue 
bow, because it is occasioned by reflection ; and for the same 
reason, when the plain glass is applied to a reflecting side, we 
can have no streaks that belong to a critical red bow, because it 
originates at the intromitting surface. With the assistance of 
