Volume 1
A text-book of experimental psychology : with laboratory exercises / by Charles S. Myers.
- Charles Samuel Myers
- Date:
- 1911
Licence: Public Domain Mark
Credit: A text-book of experimental psychology : with laboratory exercises / by Charles S. Myers. Source: Wellcome Collection.
105/370 page 85
![the dark-adapted eye (save with red and at the fovea, when it is absent), but which to the bright-adapted eye appears in the same hue as the original sensation. This is known as the ‘ positive after-image,’ and is generally attributed to a persistent activity of the once excited retinal area. The separate after-effects of the cones and rods, which have just been described, require special apparatus and ex¬ perience for satisfactory observation. But the more steady and lasting positive after-image may be observed without difficulty (exp. 72). When the eyes are closed or placed in darkness, after they have fixated a white patch, for not too long a period, a positive after-image is obtainable, provided that the back¬ ground on which the patch has rested be not too dark, and that the margins of the patch be not sharply defined. If these conditions are not fulfilled, a halo surrounds the after-image, and the latter is dark or ‘ negative ’ in character (exp. 54). Moreover, the after-image of a bright object, eg. the sun, becomes positive or negative, according as the ground on to which it is projected is darker or brighter than itself. A similar reversal is said to occur in the case of coloured after-images. Under certain conditions they appear to be positive or negative according as they are projected on to a black, or on to a white (or grey) background. Thus we see that a very close relation exists between positive and negative after-effects. Indeed it would appear that negative after-images are merely the outcome of simultaneous contrast playing upon positive after-images. But further experimental evidence is necessary before we can regard this as an assured fact. Before a coloured or colourless positive after-image dis¬ appears it often passes through different colours. This play or ‘ flight ’ of colours is doubtless due to the complex nature of our visual sensations, which are each the resultant of more elementary processes. The after-effects of these components vary in duration, and hence produce the coloured waning of the positive after-image (exp. 72).]](https://iiif.wellcomecollection.org/image/b3135984x_0001_0105.jp2/full/800%2C/0/default.jpg)
