general principle,
he set distinctly before him, and never once lost sight of, the explicit
object of his search. His imagination, now unreined, indulged itself in
the creation and invention of various hypotheses. The most plausible, or
perhaps the most fascinating, of these was then submitted to a rigorous
scrutiny; and the moment it was found to be incompatible with the
results of observation and experiment, it was willingly abandoned, and
another hypothesis submitted to the same severe ordeal. By thus
gradually excluding erroneous views and assumptions, Kepler not only
made a decided approximation to the object of his pursuit, but in the
trials to which his opinions were submitted, and in the observations or
experiments which they called forth, he discovered new facts and arrived
at new views which directed his subsequent inquiries. By pursuing this
method, he succeeded in his most difficult researches, and discovered
those beautiful and profound laws which have been the admiration of
succeeding ages. In tracing the route which he followed, it is easy for
those who live under the light of modern science to say that his
fancies were often wild, and his labour often wasted; but, in judging of
Kepler's methods, we ought to place ourselves in his times, and invest
ourselves with the opinions and the knowledge of his contemporaries.

In the infancy of a science there is no speculation so absurd as not to
merit examination. The most remote and fanciful explanations of facts
have often been found the true ones; and opinions which have in one
century been objects of ridicule, have in the next been admitted among
the elements of our knowledge. The physical world teems with wonders,
and the various forms of matter exhibit to us properties and relations
far more extraordinary than the wildest fancy could have conceived.
Human reason stands appalled before this magnificent display of creative
power, and they who have drunk deepest of its wisdom will be the least
disposed to limit the excursions of physical speculation.

The influence of the imagination as an instrument of research, has, we
think, been much overlooked by those who have ventured to give laws to
philosophy. This faculty is of the greatest value in physical inquiries.
If we use it as a guide, and confide in its indications, it will
infallibly deceive us; but if we employ it as an auxiliary, it will
afford us the most invaluable aid. Its operation is like that of the