they acquire an elasticity and a strength, for the maintenance of which the most attenuated vapor affords sufficient nourishment.
However incomplete this summary of Plutarch's voluminous tractate, it will serve to convey some idea of the state of astronomy and physics among the Greeks. In it we look in vain for simple recognition of the facts, or for any just apprehension even of the most elementary principles. Approaching their inquiries with foregone conclusions, they had decided the causes of phenomena long before they came fairly within range of them.
The point therefore is, not merely what we see, but also how we see; we must be able to critically examine what we have seen, and, above all, we must be able to recognize those features of the object which are of importance. And, as in the foregoing examples we have shown that in the domain of science mere seeing was not the strong point of the ancients, so it can be proved that they were even less distinguished for reflex seeing. By way of antithesis to a generally-received proverb, we may with more justice, though less poetry, declare that the simplicity of the child's understanding dwells on what is unimportant, but commonly passes by unnoticed what is really of moment. The senses, it is true, supply the material—the conscious, or mediate substructure—for the grandest systems of thought; but yet in their further development they must be subject to the action of the culture to which they themselves gave rise. Though at first they were our preceptors, now they are oftentimes our pupils. In seeing we have, perhaps, more need of the understanding than of the eye, just as in walking we could better dispense with strong legs than with sound lungs. The disciplined eye, though of feeble power, descries more objects difficult to be discerned than the strong but unpractised organ. This is true of the microscope and telescope as well as of the naked eye; and the student of Nature to-day, even with the imperfect instruments of his predecessors, sees much more than they. Who is there that has not innumerable times had experience of the dependence of the senses on the understanding, in the fact that, when he is intent on seeing a definite object, his eye becomes almost insensible to all other objects? Thus, one who is searching in a garden for red berries is quite unconscious of the blue berries which stand side by side with the red.
We have in German a term which very happily expresses the faculty, possessed by the most eminent of scientific geniuses, of discovering the important phases of ordinary phenomena: such men are said to have "Blick." Have we not an instance of a higher visual faculty, exalted not only by genius, but also by comprehensive knowledge, when a Gauss was led by the glistening of the windows of a church-tower which he was observing with his telescope to the idea of his heliotrope—an instrument without which no accurate triangulation is nowadays ever thought of; or when a Rittenhouse, in the
