sections, I remarked in all, in viewing through
them the flame of a candle or the lead of window panes, that
everything appeared double, though with images not very distant from
one another. Whence I understood the reason why this substance, though
so transparent, is useless for Telescopes, when they have ever so
little length.

21. Now this double refraction, according to my Theory hereinbefore
established, seemed to demand a double emission of waves of light,
both of them spherical (for both the refractions are regular) and
those of one series a little slower only than the others. For thus the
phenomenon is quite naturally explained, by postulating substances
which serve as vehicle for these waves, as I have done in the case of
Iceland Crystal. I had then less trouble after that in admitting two
emissions of waves in one and the same body. And since it might have
been objected that in composing these two kinds of crystal of equal
particles of a certain figure, regularly piled, the interstices which
these particles leave and which contain the ethereal matter would
scarcely suffice to transmit the waves of light which I have localized
there, I removed this difficulty by regarding these particles as being
of a very rare texture, or rather as composed of other much smaller
particles, between which the ethereal matter passes quite freely.
This, moreover, necessarily follows from that which has been already
demonstrated touching the small quantity of matter of which the bodies
are built up.

22. Supposing then these spheroidal waves besides the spherical ones,
I began to examine whether they could serve to explain the phenomena
of the irregular refraction, and how by these same phenomena I could
determine the figure and position of the spheroids: as to which I
obtained at last the desired success, by proceeding as follows.

[Illustration]

23. I considered first the effect of waves so formed, as respects the
ray which falls perpendicularly on the flat surface of a transparent
body in which they should spread in this manner. I took AB for the
exposed region of the surface. And, since a ray perpendicular to a
plane, and coming from a very distant source of light, is nothing
else, according to the precedent Theory, than the incidence of a
portion of the wave parallel to that plane, I supposed the straight
line RC, parallel and equal to AB, to be a portion of a wave of light,
in which an infinitude of points such as RH_h_C