PROCTOR, ON LIGHT. 163 
ness of matter required to reflect a ray. We know that in 
a soap-bubble we often see patches so thin that they do not 
reflect light, though they are still possessed of two surfaces. 
Faraday observed that some of the gold leaves he experi- 
mented with, when reduced very thin by chemical means, 
lost part of their reflecting power, though they continued to 
be free from any material injury to their surface or integrity ; 
and the proof that some depth of matter, or, as Faraday ex- 
presses it, more than one thickness of atoms, is concerned in 
ordinary reflection. It is interesting to speculate upon the 
nature of the phenomena, and the motions of atoms which 
take place in reflection, and upon the influence of this neces- 
sary thickness of matter. Is the luminous wave only re- 
flected in one phase of an undulation? If matter at some 
depth, however small, beneath the surface, continues to reflect 
light, at what depth does it cease to do so? Does it ever 
cease to do so? Or does the transmitted ray, as it speeds on 
its journey, always send back a beam in the opposite direc- 
tion ? 
Different kinds of reflecting surfaces have different appear- 
ances; this is probably due in some measure to the effect 
produced upon the light by its passage into and out of that 
thickness of matter which is concerned in ordinary reflection. 
Of homogeneous matter we have opaque and transparent, 
the former giving metallic lustre, the latter vitreous. As a 
general rule, if not a universal, we find the more nearly a 
substance approaches the metals im opacity the more it 
resembles them in the nature of its lustre. Thus, sulphurets 
are in many cases very nearly opaque, and very like metals 
in the nature of their lustre. Carbon in its opaque form 
is a brilliant steel gray, while its transparent form has the 
vitreous lustre. 
A micaceous or pearly lustre is the result of the super- 
position of a number of films of transparent material, the 
reflection from the first surface being added to by the reflec- 
tions from the subjacent surfaces. 
I use the word micaceous in preference to pearly, because 
the latter word so often is understood to mean iridescent, like 
mother-of-pearl ; but the lustre now spoken of is free from 
prismatic colours. You will see it nicely illustrated in the 
specimen I exhibit, which is a little circular piece of mica, 
which, by heating, has been split into thin lamin at the 
edges. These laminz, when taken separately, are still trans- 
parent; but the great number of them, with air between each, 
scarcely admit the passage of any light, but reflect a great 
deal; while the middle of the disc, which contains the same 
