correspond to the realities; their mental pictures misrepresent. The man of science imagines—frames a view as the initial step of all his procedures; and then, by the mental processes of comparison, reasoning, inference, proof—guided by observation and experiment—he strives to give truth to his view; that is, to harmonize it with facts, and all its parts with each other. Our writer says that science starts with observation and experiment, but the real starting-point is farther back. A mental representation must be made before it can be verified. A certain state of things is conceived or put together in thought, and is called an hypothesis; and then observation and experiment are appealed to, to test the correctness of the representation—the truthfulness of the mental picture. Science is not merely seeing with the eye or fumbling with instruments—any blockhead can do these—but it is to reconstruct Nature in thought, representing all her diverse objects, subtile relations, and complexities of change, so truly, that by every test the representation shall answer to the verities. To do this, the imagination or image-forming faculty comes into incessant play. And more than this, the genius of the discoverer depends, first of all, upon the vividness of his imagination and the power of keeping his pictures steadily before the mind's eye until their errors are detected or their accuracy established. The work of science, in fact, consists, from first to last, in the verification of mental pictures. The scientific man must be fertile in imaginative resources, but stern in his rejection of views that cannot be adjusted to facts. The poet has no such discipline, for his object is not truth. The theologian has no such discipline, for he cannot submit his views to observation and experiment, so as to test their congruity with the objective world and with each other. The picturing faculty is employed by all minds, but only the trained scientist makes it subservient to the true understanding of the order of things around.
Sufficient has been said to show that imagination is indispensable to science; but it may be asked, "If observation and experiment are the means of science for controlling the imagination, and if they furnish the conditions of its valid exercise, why prolong the vision beyond the line of experimental evidence?" The reply is, that senses and instruments are imperfect, and their indications require to be supplemented by reason. They break down at a certain point, but that point is very far from being the limit of Nature. As experiments are perfected, the line of sensible demonstration is pushed backward constantly, disclosing a continuous order. It is a right of reason and a legitimate procedure of science, to pursue this order, if the explication of known phenomena require it. The results, of course, must conform to what is established—must harmonize with all that observation and experiment have gained; but thought may be compelled to go far deeper than experiment for the explanation of facts already known.
To make this statement more concrete, let us take the very case put by Prof. Tyndall—the ultimate constitution of matter. By various lines of proof, the physicist is brought to the conclusion that there are such things as amazingly-minute physical units which he calls molecules. In their smallness they are far beyond the border of all sensible observation; but he is driven to the conclusion that they exist as realities, and he has to represent them in thought. He mentally pictures a molecule as the smallest particle of matter that can exist separately and retain its physical properties. Prof. Thompson finds physical and mathematical evidence pointing down to the actual size of molecules. From this he infers that those of water have diameters that fall within the limits of1⁄250000000 an 1⁄500000000of an inch; and adds that, if
