THE SKY. 99 



offered, if the editor cared to accept it, to send him a con- 

 tribution on the subject here presented. 



I 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 !N attire, 

 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," in virtue of which they were sometimes 



