*59 
the Cause of Coloured Concentric Rings. 
To see this still better, take an equilateral prism with a slip 
of plain glass held under it as before, and turn the axis of 
the prism gradually down at the left, till the vertical separa- 
tions of the panes of glass in the window, are equally tinged 
with the horizontal ones. Let a h and be figure 10, represent 
one of the angles made by the meeting of the divisions be- 
tween the panes cf glass ; then bring the bow streaks into the 
direction A B, and draw them gradually over the corner b , 
this motion will cause the streaks to be successively cut by 
the adventitious colours, but you will at the same time observe 
these colours to remain confined to the individual place over 
which they pass, and to produce no other effect than what 
must result from a mixture of their tinge, with the particular 
colour of the streaks at the place of their meeting. The 
corner a b c will remain perfectly single, which plainly proves 
that the adventitious colours, not being caused by critical 
separation, cannot produce streaky phenomena, whereas if 
they could diffuse themselves, we ought to see at least 5, 6, 
or 7 coloured angular figures, parallel to each other, as 
represented at d ef. 
LVIL Remarks on Colours supposed to be produced by thin Plates 
or Wedges of Air. 
First remark. In the 39th article of my paper it has been 
shown, that coloured appearances, such as streaks, cannot be 
seen between the plain surfaces of two parallel pieces of glass 
applied to each other ; if an objection should however be 
made to this, by showing an experiment with two supposed 
plain surfaces of glass in contact, where irregular streaks, or 
flashy appearances may be seen, I shall be authorized to 
Y 2 
