ray is the most refracted. One consequence of this merits a passing
notice. Pour water and bisulphide of carbon into two cups of the same
depth; the cup that contains the more strongly refracting liquid will
appear shallower than the other. Place a piece of Iceland spar over a
dot of ink; two dots are seen, the one appearing nearer than the other
to the eye. The nearest dot belongs to the most strongly refracted
ray, exactly as the nearest cup-bottom belongs to the most highly
refracting liquid. When you turn the spar round, the extraordinary
image of the dot rotates round the ordinary one, which remains fixed.
This is also the deportment of our two disks upon the screen.


§ 5. _Polarization of Light explained by the Wave Theory_.

The double refraction of Iceland spar was first treated in a work
published by Erasmus Bartholinus, in 1669. Huyghens sought to account
for this phenomenon on the principles of the wave theory, and he
succeeded in doing so. He, moreover, made highly important
observations on the distinctive character of the two beams transmitted
by the spar, admitting, with resigned candour, that he had not solved
the difficulty, and leaving the solution to future times. Newton,
reflecting on the observations of Huyghens, came to the conclusion
that each of the beams transmitted by Iceland spar had two sides; and
from the analogy of this _two-sidedness_ with the _two-endedness_ of a
magnet, wherein consists its polarity, the two beams came subsequently
to be described as _polarized_.

We may begin the study of the polarization of light, with ease and
profit, by means of a crystal of tourmaline. But we must start with a
clear conception of an ordinary beam of light. It has been already