mind that Faraday's difficulty in dealing with these conceptions was
at bottom the same as that of Newton; that he is in fact trying to
overleap this difficulty, and with it probably the limits prescribed
to the intellect itself.

The idea of lines of magnetic force was suggested to Faraday by the
linear arrangement of iron filings when scattered over a magnet.
He speaks of and illustrates by sketches, the deflection, both
convergent and divergent, of the lines of force, when they pass
respectively through magnetic and diamagnetic bodies. These notions
of concentration and divergence are also based on the direct
observation of his filings. So long did he brood upon these lines;
so habitually did he associate them with his experiments on induced
currents, that the association became 'indissoluble,' and he could
not think without them. 'I have been so accustomed,' he writes,
'to employ them, and especially in my last researches, that I may
have unwittingly become prejudiced in their favour, and ceased to be
a clear-sighted judge. Still, I have always endeavoured to make
experiment the test and controller of theory and opinion; but
neither by that nor by close cross-examination in principle, have I
been made aware of any error involved in their use.'

In his later researches on magne-crystallic action, the idea of
lines of force is extensively employed; it indeed led him to an
experiment which lies at the root of the whole question. In his
subsequent researches on Atmospheric Magnetism the idea receives
still wider application, showing itself to be wonderfully flexible
and convenient. Indeed without this conception the attempt to seize
upon the magnetic actions, possible or actual, of the atmosphere
would be difficult in the extreme; but the notion of lines of force,