On the latency of negative after‐images following stimulation of different areas of the retina

INTRODUCTION. ALTHOUGH the after-images following sudden flashes of light have been the subject of careful investigations and measurements, notably by von Kries(l6), McDougall(9s), and Fr6hlich6), quantitative data regarding the after-images following longer lasting fixation of white or coloured objects are few in number and of a non-systematic nature. The earlier literature on the subject is mostly concerned with their rather complex phenomenology and its bearing on that particular theory of colour vision which in each case has inspired the investigation. It is not surprising, therefore, that cautious physiologists have been shy of attempting in this way to procure information about vision, and have turned to processes where quantitative work is at least as easy to accomplish and is more amenable to interpretation. (Cf. for example, Parsons(22), p. 114.) Yet some experiments by Juh sz(15) indicate that the latent period preceding the development of such after-images can be estimated with an accuracy sufficient for analysis. We therefore undertook a preliminary study of the problem, and, relating it to comparatively well-known anatomical and physiological facts, measured the latency of the negative after-image of a white object in different parts of the field of vision. One of us (R. G.) had recently been making comparable experiments in which "after-images of movement" were studied with reference to the area of retina on which the primary image fell (8,9). Our investigation is confined to the ordinary persistent after-image (McD ougall's "secondary image" (19), some other writers' quaternary image), which is readily seen when, after fixation of any well-defined object, the gaze is transferred to a point on a uniform background. Purkinje(23) first described the rhythmical disappearances and reappearances of these images, and their periodicity has been more recently