Manual of physiology for students and practitioners / by H. Willoughby Lyle.

  • Lyle, H. Willoughby.
Date:
1911
Catalogue details

Licence: In copyright

Credit: Manual of physiology for students and practitioners / by H. Willoughby Lyle. Source: Wellcome Collection.

Provider: This material has been provided by The University of Leeds Library. The original may be consulted at The University of Leeds Library.

    704/776 (page 680)
    brightness and colour- this phenomenon may be appreciated when a rapidly revolving carriage wheel is observed,—individual spokes are not seen, but appear to be fused. The sensation lasts longer than the stimulus which produced it, just as the sting lasts longer than the blow which excited it. Negative after-images resemble the original image in form but differ in brightness and in colour, less bright portions of the original image appear brighter in the negative after-image and vice versa. If the original image produces a colou^ sensation, the negative after-image produces the comple- mentary colour sensation. This phenomenon is more readily explained by the Hering theory of colour vision, thus: If the object seen is black it produces anabolic changes in the black-white substance of the retina, resulting in the sensa- tion of an image which is black. After anabolism has taken place katabolism must occur, the result of which is that a negative after-image is produced with the resulting sensation of white. A similar explanation may be held to account for the colour sensations which occur from the negative after- images. The sensation of green (anabolic) is followed by the sensation of its complementary colour red (katabolic), the sensation of blue followed by the sensation of its comple- mentary colour yellow. Contrast. There are two varieties of contrast to be described: Suc- cessive contrast and simultaneous contrast. The negative after- images just described are frequently spoken of as the phenomena of successive contrast. Simultaneous contrast is applied to brightness sensations and colour sensations. Referring to the sensations of bright- ness, the phenomena may be stated as follows : A neutral- grey object appears less bright when viewed upon a white background than when viewed upon a black background. Referring to the sensations of colour, the phenomena may be described as follows: A neutral-grey object, when sur- rounded by green, appears to- acquire a tint of the colour complementary to green, namely, red. A neutral-grey object, when surrounded by blue, appears to acquire a tint of its complementary colour, namely, yellow. Simultaneous contrast persists in after-images; thus, if a grey strip, which acquires a red tint when surrounded by a green square, is looked at