the Cause of Coloured Concentric Rings. 167 
and notwithstanding what has been said in explanation of 
their production, by the reflection of the transmitted rays in 
the 14th figure of the second part of my paper, it may pro- 
bably be objected, that since the space occupied by the streaks 
adjacent to the bows, is much broader than either of the bozos, it is 
conceived, that critical separation zvill not account for them . At 
a first view of this remark, it may in some measure appear to 
be justified; for if there were no other cause than merely 
critical separation, the increase of the breadth of the streaks 
could not well be accounted for. It must however be recol- 
lected, that according to what has been proved in the 47th 
and 48th articles, the modifying power of surfaces in the pro- 
duction of streaks, is added to the principle of the critical 
separation which produces the colours. 
First cause. In consequence of the reflection of the trans- 
mitted rays, from the plain surface held under the prism, it 
may already be seen in the abovementioned 1 4th figure, that 
the streaks must take up a greater extent than the bows ; for 
instance, the last reflected colour which is marked, enters the 
prism again at v 2,67 inches beyond the faintest part of the 
bow. This gives a magnified extent of 27,44 to streaks, 
that of the bow being 24,8. 
Second cause. But in order to find a greater correspondence 
between calculation and actual observation, I must repeat that 
we are not to suppose the twenty intervals between the as- 
sumed rays to be blanks. The reason why more rays were 
not introduced, was to avoid crowding the figure unnecessarily ; 
but let us take, for instance, blue rays falling on the interior 
base at ,003 of an incli from (3 towards No. 4 ; then, in order 
to be reflected so as to reach the eye, they must have the 
Z 2 
