THE SKY. 99 
offered, if the editor cared to accept it, to send him a con- 
tribution on the subject here presented. 
1 mentioned this subject, thinking that, in addition to 
its interest as a fragment of " natural knowledge/' it 
might permit of a glance at the workings of the scientific 
mind when engaged on the deeper problems which come 
before it. In the house of Science are many mansions, 
occupied by tenants of diverse kinds. Some of them 
execute with painstaking fidelity the useful work of obser- 
vation, recording from day to day the aspects of ^Nature, 
or the indications of instruments devised to reveal her 
ways. Others there are who add to this capacity for obser- 
vation a power over the language of experiment, by means 
of which they put questions to Nature, and receive from 
her intelligible replies. There is, again, a third class of 
minds, that cannot rest content with observation and ex- 
periment, whose love of causal unity tempts them perpet- 
ually to break through the limitations of the senses, and 
to seek beyond them the roots and reasons of the phenom- 
ena which the observer and experimenter record. To 
such spirits adventurous and firm we are indebted for 
our deeper knowledge of the methods by which the physical 
universe is ordered and ruled. 
In his efforts to cross the common bourne of -the known 
and the unknown, the effective force of the man of science 
must depend, to a great extent, upon his acquired knowl- 
edge. But knowledge alone will not do; a stored memory 
will not suffice; inspiration must lend its aid. Scientific 
inspiration, however, is usually, if not always, the fruit of 
long reflection of patiently "intending the mind," as 
Newton phrased it; and as Copernicus, Newton, and 
Darwin practiced it; until outer darkness yields a glimmer, 
which in due time opens out into perfect intellectual day. 
From some of his expressions it might be inferred that 
Newton scorned hypotheses; but he allowed them, never- 
theless, an open avenue to his own mind. He propounded 
the famous corpuscular theory of light, illustrating it and 
defending it with a skill, power, and fascination which 
subsequently won for it ardent supporters among the best 
intellects of the world. This theory, moreover, was 
weighted with a supplementary hypothesis, which ascribed 
to the luminiferous molecules " fits of easy reflection and 
transmission/' iu virtue of which they were sometimes 
