An Account of Some Cases of the Production ofColours, Not Hitherto Described. Thomas Young.

                                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 confident 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 <s>in gen<\s> already 
known, I must confess that it is impossible to avoid 
another supposition, which is a part of the undulatory theory,
<s>hypothes <\s> that is, that the velocity of the light is the greater,
the rarer the medium: and that there is also a condition annexed 
to the explanation of the colours of thin plates, which involves 
another part of the same theory; that is, that then one 
of the portions of the light has been reflected at the surface 
of a rarer medium, it must be supposed to be retarded 
one half of the appropriate interval; as, for instance, in 
the central black spot of a soap bubble, when the actual 
lengths of the paths very nearly coincide, but the effect is 
the same as if one of the portions had been so retarded 
as to destroy the other. From considering the nature of this circumstance,
I ventured to predict, that if the two reflections were of the 
same kind, made at the surfaces of a thin plate, of a density