VISUAL SENSATIONS. 1053 



succeed one another; the nature of the stimulation of other parts of the 

 same retina, of the other retina, and even of other sense organs. The 

 influence of abnormal conditions of the retina 1 forms an important 

 division of the subject. In addition to visual sensations arising in the 

 usual way from stimulation by light, those due to mechanical or electrical 

 stimulation of the eyeball have also to be considered. 



Sensations of light are capable of being arranged in a continuous 

 series, passing from the deepest black or dark to the brightest white or 

 light ; the sensations are said to differ from one another in brightness 

 or luminosity. 



Sensations of colour, on the other hand, differ from each other in 

 three ways, in colour-tone or hue, in brightness or shade, and in 

 saturation or tint. 



By degree of saturation is meant the extent to which a sensation of 

 colour is fused with one belonging to the black-white series. Spectral 

 colours have a high degree of purity or saturation, but it is possible to 

 experience sensations of more than spectral saturation (see p. 1059). 



The three characteristics of colour sensations correspond to three 

 differences in the physical nature of the light stimulating the eye, namely, 

 wave-length, intensity, and complexity. The correspondence, however, 

 is not exact, and it is important to use the three physical terms only 

 when speaking of the stimulus, and to reserve the terms, colour-tone, 

 brightness, and saturation or purity for the sensations. It is often 

 useful to follow the example of painters, and to speak of the colours 

 depending on long wave-length as warm, and of those of short wave-length 

 as cold colours. 



Colour-tone, brightness, and saturation do not vary independently of one 

 another. Briicke 2 found that addition of white light to a coloured light alters 

 its colour-tone, changing it towards red, and ascribed this to a preponderance 

 of long- wave rays in white light, which is not noticed under ordinary conditions, 

 but becomes noticeable when the light is added to another colour. Eood, 3 on 

 the other hand, found that the only colour which remained unchanged was a 

 greenish yellow ; those on the long- wave side become more red on diminution 

 of saturation, while those on the short-wave side deviated towards violet. He 

 notes that all the changes were such as would be produced by the addition of 

 violet. Alteration of brightness affects colour- tone and saturation. This, 

 however, is closely connected with adaptation of the eye, and must be con- 

 sidered later. 



Quantitative standards.' In order to compare the sensibility of 

 one part of the retina with another, or of the same part under different 

 conditions, certain methods of exact estimation have been employed. 

 One of these is to determine the mimimal stimulus capable of being 

 perceived, the threshold of sensibility. In the use of colourless light 

 there is only one such threshold ; coloured light at low intensities 

 appears colourless (see p. 1079), and consequently one has here to 

 distinguish between the point at which the coloured light is seen as 

 light, the absolute threshold, and the point at which its colour can be 

 recognised, the specific or chromatic threshold. The interval between 



1 For the sake of brevity, the word "retina" is here and elsewhere frequently used 

 instead of " cerebro-retinal apparatus." Since we are almost wholly ignorant in what 

 part of the sensory system the changes determining the specific nature of a sensation take 

 place, the latter term would be more accurate. 



2 Sitzungsb. d. Jc. Akad. d. Wissensch., Wien, 1865, Bd. li. Abth. 2, S. 461. 



3 Am. Journ. Sc., New Haven, 1880, vol. xx. p. 81. 



