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.

Sec. 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 a 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.