364 



COLOUR 



COLOUR-BLINDNESS 



Colours vary in hue, in purity, and in luminosity. 

 The hue determines the name of the colour e.g. 

 vermilion, scarlet ; the purity or absence of admix- 

 ture with white light determines its richness ver- 

 milion reflects 80 per cent, of red light mixed with 

 20 of white ; the luminosity or brightness deter- 

 mines the shade or tone of colour. Interfere with 

 any of these and the impression produced upon the 

 eye is modified. Take for example a definite red 

 light, saturated or unadulterated with white light, 

 such a red as may be found in the spectrum ; pro- 

 gressively lower its purity by mixing it with white 

 light it becomes brighter, but passes through light 

 red and pink to pinkish-white tints ; lower its lum- 

 inosity by mixing with black it passes through 

 terra-cotta tones to brown, which is a dull red ; 

 interfere with both purity and luminosity by mixing 

 with various greys it goes through russets and 

 maroons, the so-called tertiary colours. Any colour 

 in nature can be matched either by a spectral 

 colour or by a purple, treated in this way ; and for 

 each such modification of the original colour there 

 Avill be a different complementary, which when 

 mixed with it forms not a white but a grey. 



The colour of transparent objects is due to 

 Selective Absorption. A red object seen through 

 greenish-blue glass appears black ; greenish-bine 

 glass is opaque to the light from the red object. 

 Hold the same piece of glass up to the sky, and 

 the red lights, which are components of the white 

 light of day, are cut off; what passes through 

 produces a sensation of greenish-blue. The red, 

 which is cut off by absorption, and the greenish- 

 blue, which passes through, are complementary 

 to one another both being really complex, not 

 monochromatic. The colour of a transparent body 

 will also apparently depend upon the thickness of 

 the layer examined : a thin layer of iodine- vapour 

 absorbs all the constituents of visible white light 

 except blue and red ; it therefore appears in day- 

 light to be purple ; a thicker layer effects the com- 

 plete absorption of the red but not that of the 

 blue, and a thicker layer of iodine-vapour there- 

 fore appears blue. If looked at in red light, a 

 thin layer of iodine-vapour appears red, while a 

 thick layer will present the blackness of opacity. 



Before a non-luminous object can be seen 

 otherwise than by transmitted light it must reflect 

 light ; if it reflect none it will appear black ; a 

 dustless pool in a mountain-hollow, a liquid in a 

 deep black vessel, may reflect no light to the eye 

 of the observer, and will appear black. (Black is 

 the negation of colour, because it implies that 

 there is no sensation of light ; gray, produced by 

 mixing white and black, is white deficient in lum- 

 inosity). Let the pool become turbid, and there 

 will be some light reflected towards the observer. 

 A coloured liquid in a deep black vessel will have 

 its colour revealed by sprinkling a white powder 

 into it. White light (daylight) enters the liquid ; 

 it is reflected in all directions by the white powder ; 

 but it is in part absorbed by the liquid, which 

 accordingly appears coloured. Of precisely the 

 same kind is the reflection of light by a solid 

 object. Bodies allow light to traverse them to a 

 very small depth, and then the light is, by internal 

 reflection, turned back in all directions; absorption, 

 meanwhile, comes into play, and the result is that 

 the object appears to have a definite colour, the 

 purity of which is marred by surface-reflection. 

 The white light reflected from the surface of a 

 metal masks its true colour, which is brought out 

 by repeated reflection. Gold is deep orange ; 

 copper, scarlet ; silver, yellowish-bronze ; brass, a 

 rich golden red. 



If the light supplied to an object do not contain 

 those kinds of light which it can reflect, the object 

 will appear black or colourless ; a bunch of flowers 



looked at by the yellow light of burning sodium 

 or of a spirit-lamp with common salt in the wick, 

 will all appear black or colourless except those 

 which are yellow. Surface-reflection modifies the 

 result. 



There is one class of cases in which colour is 

 not due to absorption. A haze is blue if its 

 particles be fine enough : if it be composed of 

 coarse particles it at once reflects white light in 

 all directions ; but fine particles cause repeated 

 reflection, and at each reflection the reflected light 

 becomes bluer ; because those rays which would 

 have been most refracted ( the blue and violet ) are 

 in fact most largely reflected. The colour of the 

 sky is that of a haze, reflecting light downwards ; 

 if there were no dust-haze or water-haze above us, 

 the sky would be black. The light which is not 

 reflected from such a haze is either transmitted 

 through it, yellower or redder in colour, or else 

 it is entirely absorbed. The sun thus appear* 

 yellower than it would do if our atmosphere did 

 not intervene. See also DICHROISM, INTERFER- 

 ENCE, IRIDESCENCE, LIGHT, OPTICS, PHOSPHOR- 

 ESCENCE, PLEIOCHROISM, RAINBOW, SPECTRUM, 

 DYEING, and the articles on the several colours ; 

 and consult also Colour by A. H. Church (Lond. 

 1887), and Colour by C. T. Whitmell (Cardiff: 

 1888). For Heraldic Colours, see HERALDRY; for 

 the Ecclesiastical Colours, VESTMENTS ; and for 

 Colour in Animals, PIGMENTS OF ANIMALS. 



Colour-blindness, a term introduced by Sir 

 David Brewster to denominate a defect of vision 

 owing to which some persons are unable to dis- 

 tinguish certain colours correctly. It is also called 

 Achromatopsia (Gr. ) and Daltonism, from Dal ton 

 the chemist, who suffered from the defect, and who 

 gave the first detailed description of it (1794). Of 

 this defect there are several degrees, classified as 

 follows (by Holmgren of Upsala) : (1) Total colour- 

 blindness, where there is no perception of colours as 

 such, but only of gradations of light and shade ; 

 (2) Complete partial colour-blindness, where some- 

 bright colours, different in different cases, are con- 

 fused with each other, though other colours are 

 correctly perceived ; ( 3 ) Incomplete partial colour- 

 blindness, where bright colours are recognised, but 

 more delicate shades are confused. The first form 

 is rare, and generally, perhaps always, associated 

 with other defects in the eyes ; the third is prob- 

 ably common, though not of great importance ; to* 

 the second attention will mainly be directed here. 



With regard to the classification of the cases of 

 complete partial colour-blindness authorities are- 

 not agreed ; the important practical point is that 

 in the vast majority red and green are the colours 

 confused. Some confuse a bright red with a green 

 that appears to a normal eye a much lighter colour ; 

 some with a green that appears darker. But if 

 suitable tests be applied it will be found that they 

 do not distinguish red and green as such. Experi- 

 ence, however, and observation of the different 

 apparent brightness of ordinary reds and greens, 

 enables them to distinguish between them in most 

 cases with wonderful accuracy, so that they may 

 remain unconscious of their defect till some strik- 

 ing mistake, or the application of a systematic 

 test, reveals it. Cases of colour-blindness for 

 yellow and blue, if they occur at all, are extremely 

 rare. Experience proves that this defect is gener- 

 ally hereditary, and is quite incurable. The eyes 

 may be, and usually are, perfect in every other 

 respect ; no difference has been detected in their 

 structure, either during life or after death ; so the 

 cause of their defective perception remains abso- 

 lutely unknown. 



Numerous careful and extensive researches both 

 in various countries of Europe and in the United 

 States have shown that this defect is present- 



