Figure 211

Three types of colour receptors or cones identified by microspectrophotometry. From Dartnall et al. (1983).

an inability to perceive blue or yellow, combined with the ability to see red and green. According to the Young-Helmholtz theory, the obvious way to try to explain the fact that red-green deficiency is the commonest form of colour blindness is to argue that the medium- and long-wavelength cone types are more likely to be damaged or missing than are the short-wavelength cones. That is actually the case (Sekuler & Blake, 1994). There are rarer cases in which the short-wavelength cones are missing, and this disrupts perception of blue and yellow. However, this is not a complete account of colour deficiency, as we will see shortly.

The Young-Helmholtz theory fails to explain negative afterimages. If you stare at a square of a given colour for several seconds, and then shift your gaze to a white surface, you will see a negative afterimage in the complementary colour. For example, a green square produces a red after-image, whereas a blue square produces a yellow afterimage.

Business Correspondence

Business Correspondence

24 chapters on preparing to write the letter and finding the proper viewpoint how to open the letter, present the proposition convincingly, make an effective close how to acquire a forceful style and inject originality how to adapt selling appeal to different prospects and get orders by letter proved principles and practical schemes illustrated by extracts from 217 actual letter.

Get My Free Ebook

Post a comment