PROCTOR, ON LIGHT. 161 
of which A, the teeth, point from the eye; a considerable portion 
of light falls under the teeth, and is thus hidden from view ; 
in the other case, at B, the eye sees principally the under or 
shady side of the teeth. The fourth eye is represented as 
looking at a powder, which, from its irregular nature, may be 
supposed to combine those various actions and many others 
tending more or less to detract from the amount of light 
entering the eye by direct reflection; there may be just as 
much reflected, but part being reflected from one particle to 
another, is more or less lost to sight. But if the particles of 
this powder are transparent, a portion of light is transmitted 
through several particles, one after another, and from the 
surface of each of these a portion is reflected, adding to the 
general luminosity. Probably the great reflecting power of 
white powders is partly due to some of the particles receiving 
the incident light at such an angle that the transmitted por- 
tion undergoes total reflection from their under side. I shall 
have occasion to notice total reflection again presently, and 
in the mean time I may remark that we should expect, from 
what I have been just pointing out, that a finely laminated 
material, such as this specimen of mica, would have the 
greatest possible reflecting power. I shall draw attention to 
this laminated mica again in speaking of lustre, and you will 
have the opportunity of observing that it really is a very 
powerful reflector. 
Reverting to the powder, we may say that, with a certain 
degree of fineness, the quantity of light reflected from the sub- 
jacent surfaces depends upon the absorbing power—the more 
absorbing power, the less light. 
Thus we conclude that bodies become lighter coloured by 
powdering, if the absorbing power is so small that the reflec- 
tion from the subjacent surfaces more than compensates for 
the loss of reflection from the breaking up of the primary 
surface ; and they become darker if the absorbing power is so 
great that the reflection from the subjacent surfaces does not 
compensate for the loss of reflection caused by the breaking 
up of the primary surface. 
So far I have endeavoured to make our progress slow and 
sure, but I wish to lead you through a variety of other con- 
siderations which would render my paper too lengthy if treated 
in the same careful manner; I will, therefore, treat the remain- 
der of the subject more briefly, more lightly, indulging in more 
speculation and less examination, and, I trust, it will be 
equally suggestive and less tedious than that which is 
ast. 
What is light? Undulation in luminiferous ether. What 
