Dr. Young’s Lecture on 
42 
somewhat similar, is by no means, as Newton imagined, iden- 
tical with the production of those of thin plates. 
Corollary iv. Of Blackness. 
In the three preceding corollaries, we have considered the 
fefracting and reflecting substances as limited by a mathema- 
tical surface; but this is perhaps never physically true. The 
ethereal atmospheres may extend on each side the surface as 
far as the breadth of one or more undulations ; and, if they be 
supposed to vary equally in density at every part, the partial 
reflections from each of the infinite number of surfaces, where 
the density changes, will very much interfere with each other, 
and destroy a considerable portion of the reflected light, so that 
the substance may become positively black; and this effect may 
take place in a greater or less degree, as the density of the 
ethereal atmosphere varies more or less equably; and, in some 
cases, particular undulations being more affected than others, 
a tinge of colour may be produced. Accordingly, M. Bouguer 
has observed a considerable loss of light, and in some instances 
a tinge of colour, in total reflections at the surface of a rarer 
medium. 
Corollary v. Of Colours by Inflection. 
Whatever may be the cause of the inflection of light passing 
through a small aperture, the light nearest its centre must be 
the least diverted, and the nearest to its sides the most : ano- 
ther portion of light falling very obliquely on the margin of the 
aperture, will be copiously reflected in various directions; some 
of which will either perfectly or very nearly coincide in direc- 
tion with the unreflected light, and, having taken a circuitous 
