3° 



HARDWICKE'S SCIENCE-GOSSIP. 



yellow light form white, whereas blue and yellow 

 pigments form green. Now we have seen that the 

 colour of a pigment is clue to the absorption which 

 light undergoes in passing through the surface layers, 

 and in being reflected back from the interior. We 

 may represent the surface layer of a blue pigment 

 by a piece of blue glass placed on white paper ; if 

 we now place a piece of yellow glass over it, what 

 will be the colour of the paper ? The blue glass 

 transmits violet, blue, and green rays ; the yellow 

 glass transmits green, yellow, orange, and red rays ; 

 hence the only rays which the two glasses transmit 

 in common are green rays. Therefore the paper will 

 look green when covered by both glasses. 



This is a model of what takes place when a mixture 

 of blue and yellow pigment is spread over a white 

 surface ; green is the only colour which is not 

 absorbed by either of them. 



Dove devised a simple piece of apparatus for 

 comparing coloured light mixtures with pigment 

 mixtures. He covered an aperture in a piece of 

 card, half with one coloured glass and half with 

 another, and caused the images of the two halves to 

 overlap by examining the aperture through a piece of 

 doubly-refracting spar. In the overlapping of the 

 "ordinary" image of the one half by the "extra- 

 ordinary " image of the other, a true mixture of the 

 lights was obtained. He afterwards observed the 

 colour of the two pieces of glass placed one over the 

 other. Dove's results show that coloured light 

 mixtures of red and green are yellow, whilst pigment 

 mixtures of the same colours are black, or very dark 

 green. Blue-green and dark purple light form a pale 

 blue-green, whereas the same colours in pigments 

 form a dark violet. In fact, pigments are always 

 darkened by mixture on the palette, for by each 

 successive mixture the absorption of some further 

 rays is effected, and thus we have a continual process 

 of subtraction ; each admixture is a stride towards 

 blackness. 



There are two accepted theories of coloured 

 vision — the Young - Helmholtz theory, and the 

 Hering theory. The former, which was published 

 by Young in 1802, supposes that there are three 

 colour sensations which are called forth in different 

 degrees by rays belonging to different regions of the 

 spectrum. One of these sensations is a red sensation, 

 which is excited most by certain red rays, but is 

 nevertheless affected in a certain varying degree by 

 other rays, the effect being less the farther the rays 

 arc from the red, until in the blue region of the 

 spectrum it is scarcely appreciable. Besides the 

 fundamental red sensation, Young supposed there to 

 be two other primary sensations, one being aroused 

 to a maximum by certain green rays, and the other 

 by certain blue or violet rays, but both, like the red 

 sensation, being more or less affected by rays of 

 every refrangibility. Helmholtz represented this 

 varying capacity possessed by rays in different 



regions of the spectrum for awakening any one of 

 the three sensations, by three curves (Fig. 14), the 

 height of any point in any of the curves representing 

 the extent to which the particular sensation is aroused 

 by a ray belonging to that part of the spectrum. 

 When the three sensations are aroused in equal 

 proportions, we experience the sensation of white 

 light ; this is shown to be the case by rotating a 

 disc with red, green, and blue sectors on Maxwell's 

 colour-top, when the whole will appear grey. We 

 have seen that yellow and blue light, when mixed, 

 makes a white or grey ; the reason for this is easily 

 understood on referring to the diagram (Fig. 14), for 

 there it is seen that a yellow ray affects both red and 

 green primary sensations, whilst a blue ray affects 

 both green and blue sensations, so that between the 

 two we have the three factors required for giving rise 

 to the sensation of white. By the diagram it is seen, 

 too, that a yellow ray awakens the red and green 

 sensations in about equal degrees. Now when a 

 disc, half green and half red, is rotated, a dull yellow 

 is the result ; when the disc is made up of more red 

 than green, the tone of the combination approaches 

 orange, as might be expected. In the same way, 

 other intermediate colour sensations are produced, the 

 resultant sensation produced by any ray or number of 

 rays being determined by the relative extent to which 

 each of the three primary or fundamental sensations 

 is affected. According to this theory, black is the 

 absence of all sensation. 



The Hering theory, which is more generally 

 accepted by* 1 physiologists, is partly based on the 

 fact that we can pick out six different colours, 

 each of which appears to us to be quite distinct from, 

 and have nothing in common with, any other ; these 

 six naturally fall into three pairs, which are black and 

 white, green and red, blue and yellow. Each pair 

 then comprises two correlative and complementary 

 colours. The theory also depends on the existence of 

 what are known as "visual substances." One such 

 substance, which is of a purple colour, is found in 

 portions of the retina of most eyes. This is called 

 "visual purple," and is peculiar from the fact that, 

 under the influence of light, it loses its colour, but 

 regains it if kept in the dark. Hering's theory, on 

 analogy with the phenomenon of visual purple, supposes 

 there to be three such visual substances in the retina 

 which are acted upon by light, and which in their 

 changed chemical state in turn act on the fibres of the 

 optic nerve, giving rise altogether to six colour sensa- 

 tions. These substances are colourless, and are con- 

 stantly undergoing either a constructive or destructive 

 metabolism. One substance awakens the sensation of 

 white when it is breaking up or undergoing dissimila- 

 tion, and of black when it is being built up or undergoing 

 assimilation. Similarly the second substance gives rise 

 to a sensation of red or green, and the third substance 

 to a yellow or blue sensation, according as dissimila- 

 tion or assimilation prevails in the respective sub- 



