the Cause of Coloured Concentric Bungs. 
1% 
LXI. Of the Manner in which Rays that are Separated by critical 
Refection or Intromission come to the Eye. 
The subject of vision in general, affords so many intricate 
phenomena, that we must not be surprised if some things 
occur, that are of difficult conception. By means of the prin- 
ciple of the intromissive separation of the colours, I have 
already accounted for several appearances, that no other 
principle, not even the Newtonian fits of easy reflection, 
and easy transmission can possibly reach. If therefore, it 
should be objected to my ascribing the generation of the 
colours of the Newtonian rings, likewise to critical separa- 
tion that rings must arise from some other cause than critical 
separation, because they can be seen at the under surface of a glass 
terminated by parallel planes , (as in figure \y,) and in other 
situations in which critical separation cannot reach the eye, it will be 
necessary to examine how far this observation is well founded. 
The objection seems to convey a double assertion ; the 
first is, that in the situation of a plain glass laid upon a con- 
vex surface, no critical separation can reach the eye ; and the 
next, that if I cannot show how the rays come to the eye, the 
rings cannot be caused by critical separation. The first of 
these positions contains something taken for granted, which 
cannot be admitted. It supposes that I affirm the critical 
separation to be the sole cause of the rings, whereas I have 
plainly shown, that this separation furnishes only the colours, 
and that the modifying power of the subjacent spherical re- 
flecting surface, turns these colours into rings. Now to show 
how very different an effect may be produced, when the cri- 
tically separated rays are acted upon by the modifying power 
