Colours seen in dreams by six observers were recorded from memory and plotted on a CIE u, v, chromaticity diagram. Only about half the dreams recorded contained colour, and in those in which colour appeared the more saturated purples, blues and blue greens were absent. It is suggested that during achromatic dreams the areas of the visual cortex which seem to respond only to colour may be inoperative. The paucity of blue in dreams could be anatomically related to the small population of blue units in the colour areas of the cortex.
The historical background of the subject is given, together with a discussion of the types of scales with which attempts have been made to measure subjective~rightn~s. T~e~pparat~s uf or direct estimation experiments is described, and~he meth~of direct estlm~tlon ISexplamed m detail. In this method the observer is presented WIth .a se~les of test fi~lds m rand~m order of luminance and asked to grade them by directly estimating the magnitude of thejr apparent brightness. The method was exhaustively explored and the effec~s were found o~different group~of observers, the size of luminance step, the order of presentation~f t.he .Iummances, field. s!ze, foveal and extra foveal viewing, and the colour ofthe test field. Scaling IS dISCU.S~, and~tatlstlcal evidence showing that the directly estimated brigh~nesse~are on a ratio scale IS gl!en. FI~ld work confirmed that the apparent brightness data obtained WIth laboratory apparatus IS applicable to practical situations. The effect of the adaptation field is discussed. Bino~ular matching e~perJments were used to relate more accurately apparent brightness data to djfferent adaptation levels. Apparent brightness scales for light different adaptation levels are given which can be used for the luminance design of interiors.
MUCH has been written on the persistence of visual sensation after the light stimulus has been removed-the so-called visual after-image (Helmholtz, 1924; Duke-Elder, 1932). The after-image may be positive, in which the light and dark parts correspond to those of the object, or negative, in which the light and dark parts are reversed. Similarly, in the positive after-image of a coloured object, the colours roughly correspond to those of the object, whereas in the negative after-image complementary colours are said to appear. With a very bright stimulus, the after-image may persist for several minutes or perhaps longer, whereas under more normal conditions the duration is short, and may be studied by experiments on flicker and persistence of vision. Whilst the short-period phenomena have received considerable attention, the long-period after-images have mainly been studied qualitatively, and no comprehensive quantitative experimental work appears to have been done. Craik (1940), and later Cibis and Nothdurft (1948), have described experiments from which they conclude that after-images are of retinal origin, and Misiak and Lozito (1951) show that some central modification of afterimages occurs. To decide, however, on the mechanism of after-images, and on their relation to other phenomena of vision seems premature until more experimental data is available. Pannevis (1947; 1948) has made measurements on the brightness variation of long-period after-images, although the amount of work is not great. His method was to re-stimulate the letina and induce a negative after-image equal in apparent brightness to the positive after-image, so that no resultant image could be seen. The assumption that the positive after-image is not affected by re-stimulation is probably unjustifiable. Probably the reason for the lack of adequate quantitative data is due to the difficulty of evolving a technique for making adequate measurements. The author has found that, by using the binocular matching technique of Wright (1946), measurements could be made of the retinal illumination of one eye which caused a sensation of brightness equal to that of the positive after-image in the other eye. Also he has found that by taking a sufficient number of observations, significant quantitative data can be obtained. Work has been done with white lightt stimuli only, on a dark-adapted eye.
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