60 HENRY RUTGEES MARSHALL: the acceptance of such a view without full warrant. Let us then in the first place study the facts with which we are con- cerned, without any prejudice in favour of the view above considered which our very first examination leads us to suspect is but crudely conceived. Sec. 5. In the first place let us try to make a clear state- ment of the experiences to be studied, even though in so doing we repeat what is very familiar. Suppose that in one moment with my eyes open I have what I call a sensation of yellow, and that in a second moment I close my eyes. If in the first moment the sensation was very vivid, I may have in the second moment what we choose to call a primary- memory-image which is evidently much like the yellow sensa- tion ; or I may have a negative " after-image," which is of a sensational nature, although the colour given is the comple- ment of the yellow of the original impression. On the day following I may have a remembrance of this sensation of yellow, or of the complementary " after-image," which is as far as possible removed from actual sensation, but which in- volves an " idea " of the colour. Such are the simplest experiences of the type we are here examining, although they are relatively speaking rare ; for we do not often experience sensations which involve so little meaning. We may turn then to the more usual cases in- volving what we call perception. I hear a knock at the door ; I hear the clock strike ; I see my friend before me. But the knocking stops, the clock strokes cease, my friend leaves the room, and the original impressions are gone ; in place of them I have the " primary -memory-image " of the knocking, of the clock bell strokes, of the friend's face. This primary-memory-image is very clearly of the same general nature as the " image " we have when no perceptive process has immediately preceded. The ideas w T hich we have to-day of yesterday's perception of the knock on the door, of the clock strokes, of the face of the friend, are so clearly allied to the primary-memory-images of yesterday that we apply the same term images to these " ideas " of to-day. It appears to the writer that much light is thrown upon the nature of the presentations above described by consider- ing the nature of the coincident activities in the nervous system, and in this connexion we may make use of a diagram which may serve to simplify the description of the very com- plex phenomena which we are to study. Sec. 6. In figure 1 below I intend to represent in symbolic form the nervous system of man, which is a vastly complex
