which proved that even logicians share the common fire of
humanity. He interested me most when he became the mirror of my own
condition. Neither intellectually nor socially is it good for man to
be alone, and the sorrows of thought are more patiently borne when we
find that they have been experienced by another. From certain
passages in his book I could infer that Mr. Bain was no stranger to
such sorrows. Speaking for example of the ebb of intellectual force,
which we all from time to time experience, Mr. Bain says: 'The
uncertainty where to look for the next opening of discovery brings the
pain of conflict and the debility of indecision.' These words have in
them the true ring of personal experience. The action of the
investigator is periodic. He grapples with a subject of enquiry,
wrestles with it, and exhausts, it may be, both himself and it for the
time being. He breathes a space, and then renews the struggle in
another field. Now this period of halting between two investigations
is not always one of pure repose. It is often a period of doubt and
discomfort--of gloom and ennui. 'The uncertainty where to look for
the next opening of discovery brings the pain of conflict and the
debility of indecision.' It was under such conditions that I had to
equip myself for the hour and the ordeal that are now come.

*****

The disciplines of common life are, in great part, exercises in the
relations of space, or in the mental grouping of bodies in space; and,
by such exercises, the public mind is, to some extent, prepared for
the reception of physical conceptions. Assuming this preparation on
your part, the wish gradually grew within me to trace, and to enable
you to trace, some of the more occult features and operations of Light
and Colour. I wished, if possible, to take you beyond the boundary of
mere observation, into a region where things are intellectually
discerned, and to show you there the hidden mechanism of optical
action.

But how are those hidden things to be revealed? Philosophers may be
right in affirming that we cannot transcend experience: we can, at all
events, carry it a long way from its origin. We can magnify,
diminish, qualify, and combine experiences, so as to render them fit
for purposes entirely new. In explaining sensible phenomena, we
habitually form mental images of the ultra-sensible. There are Tories
even in science who regard Imagination as a faculty to be feared and
avoided rath