light possesses no weight, gives us the means of magnifying small
motions to an extraordinary degree. Thus, by attaching mirrors to his
suspended magnets, and by watching the images of divided scales
reflected from the mirrors, the celebrated Gauss was able to detect
the slightest thrill of variation on the part of the earth's magnetic
force. By a similar arrangement the feeble attractions and repulsions
of the diamagnetic force have been made manifest. The minute
elongation of a bar of metal, by the mere warmth of the hand, may be
so magnified by this method, as to cause the index-beam to move
through 20 or 30 feet. The lengthening of a bar of iron when it is
magnetized may be also thus demonstrated. Helmholtz long ago 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