the Cause of coloured concentric Rings. 293 
will be seen; but when afterwards the eye is gradually ele- 
vated, till by the appearance of the blue buw we find that the 
principle of the critical separation of colours is exerted, the 
streaks will become visible, and not before ; nor will they re- 
main in view when the eye is lifted higher than the situation 
in which the effects of the critical separation are visible. It is 
therefore evident, not only that the colours are furnished by 
the same cause which produces the bow, but also that they 
are modified into streaks by the plain surface under the 
prism. 
In addition to this, it must be remarked that the criterion, 
which has been successfully used in the explanation of several 
prismatic phenomena, proves that no other colours, but those 
which arise from the same source, can be modified so as to 
give streaks. The following experiment will show the foun- 
dation on which this criterion is established. 
Let there be an horizontal opening in the upper part of a 
window-shutter, of about three feet long and one foot high; 
then, if we look at it through one side of a right angled prism, 
we shall see a red bow from the highest margin of the open- 
ing, and a blue one from the lowest ; but when a plain glass 
is applied to either of the sides of the prism through which 
we see these bows, neither of them will give any coloured 
streaks. The experimenter must carefully keep the critical 
bows out of the way ; for should either of them fall upon 
those which are under examination, streaks must of course be 
seen to pass over them. 
When a spherical surface is placed under the prism, it has 
likewise been shown that coloured rings will be seen ; but 
these, like the streaks, will not be visible when the eye is 
MDCCCIX. Q q 
