the Theory of Light and Colours. 41 
which, like Newton’s, would lead us to expect the greatest irre- 
gularities in the direction of the refracted rays ; or, like Mr. 
Mi c hell’s, would require such effects from the change of the 
angle of incidence, as are contrary to the effects observed; or 
they are equally deficient with respect to both these circum- 
stances, and are inconsistent with the most moderate attention 
to the principal phenomena. 
Corollary iii. Of the Colours of thick Plates. 
When a beam of light passes through a refracting surface, 
especially if imperfectly polished, a portion of it is irregularly 
scattered, and makes the surface visible in all directions, but 
most conspicuously in directions not far distant from that of 
the light itself: and, if a reflecting surface be placed parallel to 
the refracting surface, this scattered light, as well as the prin- 
cipal beam, will be reflected, and there will also be a new dis- 
sipation of light, at the return of the beam through the refracting 
surface. These two portions of scattered light will coincide in 
direction ; and, if the surfaces be of such a form as to collect 
the similar effects, will exhibit rings of colours. The interval 
of retardation is here, the difference between the paths of the 
principal beam and of the scattered light between the two sur- 
faces ; of course, wherever the inclination of the scattered light 
is equal to that of the beam, although in different planes, the 
interval will- vanish, and all the undulations will conspire. At 
other inclinations, the interval will be the difference of the 
secants from the secant of the inclination or angle of refraction 
of the principal beam. From these causes, all the colours of 
concave mirrors observed by Newton and others are necessary 
consequences : and it appears that their production, though 
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