39% Dr. Young's Account of some Cases 
It must however be observed, that the colours are not pro- 
duced in the whole light that is transmitted through the me- 
diums : a small portion only of each pencil, passing through the 
water contiguous to the edges of the particle, is sufficiently 
coincident with the light transmitted by the neighbouring por- 
tions of air, to produce the necessary interference ; and it is 
easy to show that, on account of the natural concavity of the 
surface of each portion of the fluid adhering to the two pieces 
of glass, a considerable portion of the light which is beginning 
to pass through the water will be dissipated laterally by reflec- 
tion at its entrance, and that much of the light passing through 
the air will be scattered by refraction at the second surface. 
For these reasons, the fringes are seen when the plates are not 
directly interposed between the eye and the luminous object; 
and, on account of the absence of foreign light, even more dis- 
tinctly than when they are in the same right line with that 
object. And, if we remove the plates to a considerable distance 
out of this line, the rings are still visible, and become larger 
than before; for here the actual route of the light passing 
through the air, is longer than that of the light passing more 
obliquely through the water, and the difference in the times of 
passage is lessened. It is however impossible to be quite confi- 
dent with respect to the causes of these minute variations, 
without some means of ascertaining accurately the forms of the 
dissipating surfaces. 
In applying the general law of interference to these colours, 
as well as to those of thin plates already known, I must confess 
that it is impossible to avoid another supposition, which is a 
part of the undulatory theory, that is, that the velocity of light 
is the greater, the rarer the medium ; and that there is also a 
