sent advanced state, is chiefly
promoted. Most of the phenomena which nature presents are very
complicated; and when the effects of all known causes are estimated
with exactness, and subducted, the residual facts are constantly
appearing in the form of phenomena altogether new, and leading to the
most important conclusions."[2]

It is obvious that this is not a primary method of observation, but
a method that may be employed with great effect to guide observation
when a considerable advance has been made in accurate knowledge
of agents and their mode of operation. The greatest triumph of the
method, the discovery of the planet Neptune, was won some years after
the above passage from Herschel's Discourse was written. Certain
perturbations were observed in the movements of the planet Uranus:
that is to say, its orbit was found not to correspond exactly with
what it should be when calculated according to the known influences
of the bodies then known to astronomers. These perturbations were a
residual phenomenon. It was supposed that they might be due to
the action of an unknown planet, and two astronomers, Adams and Le
Verrier, simultaneously calculated the position of a body such as
would account for the observed deviations. When telescopes were
directed to the spot thus indicated, the planet Neptune was
discovered. This was in September, 1846: before its actual discovery,
Sir John Herschel exulted in the prospect of it in language that
strikingly expresses the power of the method. "We see it," he said,
"as Columbus saw America from the shores of Spain. Its movements have
been felt, trembling along the far-reaching line of our analysis, with
a certainty hardly inferior to that of ocular demonstration."[3]

Many of the new elements in Chemistry have been discovered in this
way. For example, when distinctive spectrums had been observed for all
known substances, then on the assumption that every substance has a
distinctive spectrum, the appearance of lines not referable to any
known substance indicated the existence of hitherto undiscovered
substances and directed search for them. Thus Bunsen in 1860
discovered two new alkaline metals, Caesium and Rubidium. He was
examining alkalies left from the evaporation of a large quantity of
mineral water from Durkheim. On applying the spectroscope to the flame
which this particular salt or mixture of salts gave off, he found that
some bright lines were visible which he had never obs