In the three preceding corollaries, we have considered the refracting
and reflecting substances as limited by a mathematical 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.