Dr. Herschel’s Experiments for Investigating 
to whatever extent the circumstances of the combination of 
the two essential surfaces may allow. The very case pro- 
posed in the objection, of a slip of plain glass laid on a sphe- 
rical surface, has been examined in the most simple form 
possible, by using for the spherical curvature, a piece of 
polished metal, to exclude all adventitious source of the ge- 
neration of colours, and employing for the contact of a plain 
surface, the base of a prism, on the inside of which, according 
to the Newtonian doctrine of the different reflexibility of 
light, it must be admitted, the colours will be critically sepa- 
rated. Then, without the least change of form, or contact of 
the two essential surfaces, it has been proved, that a diminu- 
tion of the prismatic angle, will gradually extend the visibility 
of the rings, till, even before that angle comes to a vanishing 
state, where the prism would be converted into a plain slip of 
glass, the rings may be seen in every direction, and at any 
elevation of the eye, in which they can be seen, through a slip 
of glass such as the objection supposes. As this then has been 
proved not only of rings, but of all other possible configura- 
tions, that may be caused by the rays of the critically separated 
colours, modified by reflection or radiation from curved sur- 
faces, it is evident, that an objection which asserts that such 
colours cannot be seen, contradicts the plainest and best 
established facts. 
With regard to the actual course of the rays from the very 
moment of their critical separation into colours, till they pro- 
duce the required effect, it cannot surely be expected that I 
should trace them through a most intricate complication of 
reflections from curve to curve, when it has been shown, in 
the second part of my paper, that even with streaks, which 
