Two theories have been propounded to explain the sensation of colour produced upon the retina.
The Young-Helmholtz theory teaches that there are three primary sensations—red, green, and violet. Other colours are a mixture of these sensations; white is produced when all three sensations are excited together, and black is an absence of sensation.
Hering’s theory is that there are six primary sensations arranged in three pairs—white and black, red and green, and yellow and blue. He assumes the existence of three visual substances which undergo metabolic changes when subjected to the action of light. These are the red-green, the yellow-blue, and the white-black substances. The white-black substance is influenced by all the rays of the spectrum, while the red-green and yellow-blue substances are differently influenced by rays of different wave-lengths. When all the rays together fall upon the retina, no metabolism takes place in the red-green and yellow-blue substances, but only the white-black substance is affected. Thus the white-black substance is the most active.
Any discussion as to the relative value of these theories would in this work be out of place and unnecessary.
The ordinary form of colour-blindness in human beings is the inability to discriminate between red and green. This shows that the visual power of these people is dichromatic and not trichromatic, as their power is limited to two colours, or pairs of colours, and does not extend to three.
The individuals who belong to this class of the colour-blind may be divided into two sub-classes—those who are red-blind and those who are green-blind.
Those who are red-blind do not see the red end of the spectrum, and the blue-green appears grey, though they have distinct colour vision of the parts of the spectrum on either side of the blue-green. In matching red with a green, they put a bright red with a dark green.
On the other hand, those who are green-blind see the red end of the spectrum, while the green appears to them as grey. In matching a red with a green they put a dark red with a bright green.
No absolutely undoubted cases of blue-yellow blindness have been recorded, and only one of absolute colour-blindness; but one case is not sufficient to go upon.
According to the Young-Helmholtz theory, a case in which only shades of black and white were visible would be impossible, as it would not be shades of black and white which would be seen, but shades of either red, green or blue. According to Hering’s theory, of course, absolute colour-blindness would be possible.