Six Lectures on Light | Page 6

employed this method of rendering
evident to his students the classical experiments of Du Bois Raymond
on animal electricity; while in Sir William Thomson's reflecting
galvanometer the principle receives one of its latest and most important
applications.
§ 4. _The Refraction of Light. Total Reflection._
For more than a thousand years no step was taken in optics beyond this
law of reflection. The men of the Middle Ages, in fact, endeavoured, on
the one hand, to develop the laws of the universe _à priori_ out of their
own consciousness, while many of them were so occupied with the
concerns of a future world that they looked with a lofty scorn on all
things pertaining to this one. Speaking of the natural philosophers of
his time, Eusebius says, 'It is not through ignorance of the things
admired by them, but through contempt of their useless labour, that we
think little of these matters, turning our souls to the exercise of better
things.' So also Lactantius--'To search for the causes of things; to
inquire whether the sun be as large as he seems; whether the moon is
convex or concave; whether the stars are fixed in the sky, or float freely
in the air; of what size and of what material are the heavens; whether
they be at rest or in motion; what is the magnitude of the earth; on what
foundations is it suspended or balanced;--to dispute and conjecture
upon such matters is just as if we chose to discuss what we think of a
city in a remote country, of which we never heard but the name.'
As regards the refraction of light, the course of real inquiry was
resumed in 1100 by an Arabian philosopher named Alhazen. Then it
was taken up in succession by Roger Bacon, Vitellio, and Kepler. One

of the most important occupations of science is the determination, by
precise measurements, of the quantitative relations of phenomena; the
value of such measurements depending greatly upon the skill and
conscientiousness of the man who makes them. Vitellio appears to have
been both skilful and conscientious, while Kepler's habit was to
rummage through the observations of his predecessors, to look at them
in all lights, and thus distil from them the principles which united them.
He had done this with the astronomical measurements of Tycho Brahe,
and had extracted from them the celebrated 'laws of Kepler.' He did it
also with Vitellio's measurements of refraction. But in this case he was
not successful. The principle, though a simple one, escaped him, and it
was first discovered by Willebrord Snell, about the year 1621.
Less with the view of dwelling upon the phenomenon itself than of
introducing it in a form which will render subsequently intelligible to
you the play of theoretic thought in Newton's mind, the fact of
refraction may be here demonstrated. I will not do this by drawing the
course of the beam with chalk on a black board, but by causing it to
mark its own white track before you. A shallow circular vessel (RIG,
fig. 4), half filled with water, rendered slightly turbid by the admixture
of a little milk, or the precipitation of a little mastic, is placed with its
glass front vertical. By means of a small plane reflector (M), and
through a slit (I) in the hoop surrounding the vessel, a beam of light is
admitted in any required direction. It impinges upon the water (at O),
enters it, and tracks itself through the liquid in a sharp bright band (O
G). Meanwhile the beam passes unseen through the air above the water,
for the air is not competent to scatter the light. A puff of smoke into
this space at once reveals the track of the incident-beam. If the
incidence be vertical, the beam is unrefracted. If oblique, its refraction
at the common surface of air and water (at O) is rendered clearly visible.
It is also seen that reflection (along O R) accompanies refraction, the
beam dividing itself at the point of incidence into a refracted and a
reflected portion.[4]
[Illustration: Fig. 4.]
The law by which Snell connected together all the measurements
executed up to his time, is this: Let A B C D (fig. 5) represent the
outline of our circular vessel, A C being the water-line. When the beam
is incident along B E, which is perpendicular to A C, there is no

refraction. When it is incident along m E, there is refraction: it is bent at
E and strikes the circle at n. When it is incident along _m'_ E there is
also refraction at E, the beam striking the point _n'_. From the ends of
the two incident beams, let the perpendiculars m _o_, _m'_ _o'_ be
drawn upon B D, and from the ends