Page:Transactions NZ Institute Volume 25.djvu/652

 LXXIII.—After-images.

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phenomena to which I wish to draw your attention to-night are so common that my only excuses in bringing them before your notice are—firstly, the great interest I have always taken in the subject; and, secondly, the hope that if, after hearing the notes, you say to yourselves, "We knew all that before," some other member will take up the theme and tell us a little about his experiences, or throw some further light on the question.

Every one has noticed after-images, but few realise what an important bearing they have on the discussion of memory, for, unless percepts persisted for a time, we should be unable to grasp the idea that separate perceptions—say a, b, c, d, e—form one whole. After-images form a connecting-link between percepts and revived mental images, and they probably underlie many of the lesser acts of remembering, as Sully has well pointed out. Revived mental images are more important, because they lead to greater knowledge; but a clear understanding of after-images forms a good introduction to the subject of reproductive imagination.

It will be well at the beginning of this paper to define clearly what is meant by an image in psychology. James Sully, in his Outlines, distinguishes an "image" from a "percept" by saying that a "percept" is largely presentative, while an " image " is representative.

On the other hand, considering "images" under the heading of "ideas," an "image" differs from a "concept" or "general notion," for the latter deals with a class, while the former represents a concrete object or mental picture.

But the after-images I wish to speak of to-night are physiological rather than psychological phenomena; and it would have been better to have called them "after-percepts" had not the name "after-image" been better known. After-percepts of sight are the most frequent, although I have heard or read of after-percepts derived from all the organs of sense and movement. But to-night I shall deal only with the images of the organs of sight.

These images are divided into two classes—positive and negative. By a positive image we mean that colours in the representative image are of the same kind as in the presented object, while in a negative after-image the light colours become