PROCTOR, ON LIGHT. 157 
has a dark centre while it is beyond focus, is uniformly light 
at focus, and has a bright centre when within focus, indicat- 
ing that the balsam has a higher refracting power than the 
granules; with that mounted in the gelatinous medium, you 
will find that the effects of increasing and diminishing the dis- 
tance from the object-glass are reversed, indicating that the 
starch has now a refracting power above that of the medium 
in which it is enveloped, and the markings on its surface are 
in this mounting much more distinct. No doubt similar 
attention to the mounting medium would, in many other 
cases, produce a like improvement in the appearance of the 
object. I will only adduce another example—two mount- 
ings of shell; that in gelatine shows the laminations more 
distinctly than that in balsam. 
Various other matters, more or less important and inte- 
resting to the microscopist and optician, might be treated of, 
in connection with this part of my subject, but contenting 
myself with what I have said, I will revert to some of the 
other questions raised at the beginning of my paper. 
Are white organic materials ever opaque ? 
Are all opaque substances black when in a fine state of 
division ? 
How far is blackness coincident with opacity and hetero- 
geneousness or division ? 
How far is whiteness coincident with transparency and 
heterogeneousness or division ? 
Is anything opaque? 
Is anything opaque? That is the question which ought to 
have preceded and superseded some, at least, of the others. 
We can only answer it inductively from the results of nume- 
rous observations. Glass is a transparent substance, approxi- 
mately so, that is to say; for if we look through a plate of 
glass edgeways, we find it stops a great deal of light. It is 
not so transpareut as pure water—and even water, as pure as 
it could be obtained by distillation, was proved by Professor 
Tyndall to have a blue-green colour when light was passed 
through fifteen or sixteen feet of it. A sheet of paper looked 
coloured seen through the water, but white seen through the 
same thickness of air. Is air, then, our only perfectly transpa- 
rent medium? Even air is far from transmitting all the hight 
which enters it. A comet is almost incalculably more transpa- 
rent than the earth’s atmosphere. The light of a star passing 
through hundreds of thousands of miles of a comet’s atmo- 
sphere and nucleus loses less light than in passing through 
the thin stratum of air which covers the earth ; yet even the 
