VISUAL SENSATION. The problem of visual sensation has to do with the dependence of visual experience upon certain features of the physiological visual process which lie on the so called "afferent" side of the nervous arc. These features include the elementary properties of (I) the stimulus (light or electro magnetic radiation having appropriate wave-lengths), (2) the retinal excitation, (3) the conduction along the optic nerves, and (4) certain aspects of cerebral activity. The boundary line be tween sensation and perception is somewhat vague. From the classical standpoint, perception is concerned with the manner in which various constituent factors are put together to form dis tinct psychical patterns, these patterns being regarded in general as representations of objects which are actually before the eyes. Sensation, on the other hand, is considered as furnishing the materials for such perceptual syntheses, and as being concerned more with the stimulus energy and the sense organ response than with either the brain processes or the object. However, modem studies of the manner of dependence of experience upon cerebral conditions have made rather free use of the concept of sensation.
If we regard visual sensation, from the introspective stand point, as a subjective material, out of which complex visual per ceptions are compounded, the substance of such sensation ap pears to be reducible to two kinds, colour and depth. Colour characterizes object surfaces, while depth constitutes the spatial medium which intervenes between these surfaces and the empirical eye. These elementary factors can be arranged, at various times, so as to form an indefinitely large number of characteristic per ceptual patterns, to constitute the phenomena of visual experience.
primary hues, are red, yellow, green and blue, but these have many intermediates, of which the typical ones are orange, yellow green, blue-green and purple. The total number of possible achro matic colours has been estimated at between 600 and Boo, of discriminable hues at about I so, and of different saturation values of each hue, at approximately 20. According to Titchener, the total number of discriminable colours is about 35,00o, but this number must vary widely with conditions.
The achromatic colours, or greys, can be arranged in linear order, so as to form a system which has only a single dimension. This dimension has been variously designated as one of bright ness, luminosity, value, or tint, but the term brilliance is to be preferred because of its freedom from disturbing connotations. Brilliance is at a minimum for an ideal black and is at a maximum for an ideal white. Intervening greys can be designated numeri cally by counting the number of just noticeably different achro matic colours which separate them from the ideal black. The chromatic colours may also differ from one another in brilliance, although if they differ in this only they will have identical chromas.
If we consider all chromatic colours which are possible of the same brilliance, we find that they can be ordered to form a sur face, or a two-dimensional figure. This figure will automatically include the single achromatic colour which is of the given brilli ance. Any colour within this surface can evidently be specified in terms of values along each of the two dimensions. The axes for the latter are best chosen in accordance with a system of polar coordinates, with its centre at the grey point. Radial values, measured in just noticeable steps from the centre, then represent the saturations of the colours, whereas circumferential values designate the hues.