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, and of their divergence and convergence,
guides Faraday without perplexity through all the intricacies of the
question. After the completion of those researches, and in a paper
forwarded to the Royal Society on October 22, 1851, he devotes himself
to the formal development and illustration of his favourite idea. The
paper bears the title, 'On lines of magnetic force, their definite
character, and their distribution within a magnet and through space.'
A deep reflectiveness is the characteristic of this memoir. In his
experiments, which are perfectly beautiful and profoundly suggestive, he
takes but a secondary delight. His object is to illustrate the utility
of his conception of lines of force. 'The study of these lines,' he
says, 'has at different times been greatly influential in leading me to
various results which I think prove their utility as well as fertility.'

Faraday for a long period used the lines of force merely as 'a
representative idea.' He seemed for a time averse to going further in
expression than the lines themselves, however much further he may
have gone in idea. That he believed them to exist at all times round a
magnet, and irrespective of the existence of magnetic matter, such as
iron filings, external to the magnet, is certain