The purely constructive man does not cause any such jolt. His work proceeds smoothly, uniformly, and usually with approval. But the inventive man, "his eye in a fine frenzy rolling," is visited with some vision which he cannot or will not dismiss, and which compels him to try to embody it in some form, and to continue to try until he succeeds in doing so, or gives up, confessing failure. The inventive man, having seen the vision, becomes a constructive man, and (in case he succeeds) _puts the vision which he sees into such form that other people can see it also_. It is obvious therefore that two kinds of ability are needed to produce a really good invention of any kind, inventive ability and constructive ability; and it is also obvious that they are separate, though they cooperate. Many an invention of a quality that was mediocre or even inferior in originality, novelty and scope, has been quite acceptable by reason of the excellent constructive work that was done upon it: many a book and many an essay has succeeded almost wholly because of the skilful construction of the sentences; many a picture because of the accuracy of the perspective and the mixing of the colors; many a new mechanical device because of the excellent workmanship bestowed upon it. Conversely, many a grand and beautiful conception has failed of recognition because of the poor constructive work that was done on it. But occasionally a Shakespeare has given to the world an enduring masterpiece, the joint work of the highest order of invention and the highest order of constructive skill; occasionally a Raphael has painted a picture similarly conceived and executed; and occasionally an Edison has given the world a mechanical invention, comparably wonderful and perfect. In all such cases, the start of the work was a picture on the mental retina; an image of something that was not, but might be made to be. A physical picture is actually made on the physical retina, but it cannot be recognized by the owner of the retina, unless a healthy optic nerve transmits it to his brain. Every mental picture must also be transmitted to the brain; and some mental pictures are very bright and clear. In some forms of insanity, the mental pictures are so clear that the patient cannot be persuaded that they are not physical; the patient sees a man approaching him, when there is no man approaching him; but the impression made on the patient's mind is the same as if there were. The thought of the enormousness of the consequences that have followed the appearing of some visions to men (the vision of the gun, for instance) is almost stunning, if we try to realize the small area of the brain that the vision must have covered. If a line 1/4000 of an inch wide made on the physical retina and afterwards transmitted to the brain is seen with perfect clearness by the mind, what a small area of the brain must have been covered by the original vision of the gun! Yet how vast have been its consequences!