the Cause of Coloured Concentric Rings. 171 
take up the opposite set of rings, and confining our attention to 
it, may draw the eye down again till we lose it : which will not 
be, till when the eye is nearly brought down to the level of 
the side exposed to the light. Here then we have an instance 
of rings composed of the colours furnished by critical separa- 
tion, which may be seen from the obliquity of 82® 17' 31", 
down to I suppose about 5 degrees, which gives an angular 
space of at least 77 degrees. 
To extend this range farther, I used several prisms with 
different refracting angles ; first, one where that angle was 
30 degrees ; then one with 25 ; another with 20 ; and also 
one of 9 degrees. By this successive change of prisms, it was 
ascertained, that the range of visibility increased, when a 
smaller refracting angle was used. In the last prism, the 
rings became visible at an elevation of $6° 4,3' 6 !> ; this up to 
90 on one side, and down again at the other to 5 degrees, 
gives a range of more than 138 degrees, in which these rings 
may be seen. To manage this experiment it is necessary, 
when the eye is vertical, gently to turn the mirror with the 
prism upon it half round, that the eye may then be depressed 
gradually, without interfering with the incident light. 
From these experiments it may be presumed, that were the 
refracting angle still farther diminished, it would increase 
the range at last to that of a plain glass, which therefore, I 
am authorized to look upon, as a prism with a vanishing 
refracting angle. It will be seen presently, that even this has 
been completely verified. 
As the modifying power of spherical surfaces, to render 
critically separated colours visible in every direction, is by 
these experiments established, we might take it for granted. 
