COLOUR SENSATIONS Although there are considered to be but six or seven spectrum colours—red, orange, yellow, green, blue, indigo, and violet—they melt so insensibly one into the other that there are an infinite number of distinct colours ; again, in natural objects there are innumerable colours. It has been proved, however, that there are only three fundamental colours, that give rise to all the other colours by the excitation of three sets of nerve fibrils in the retina. This theory was enunciated first by Thos. Young (" Lectures on Natural Philosophy," 1807), and further elabor ated by Clerk-Maxwell (Edinburgh Transactions, 21,275, 1855), Von Helmholtz, Abney, K8nig, and others. The curves representing the respec tive actions of the various spectrum colours, according to Konig and Dieterici, are shown in the accompanying diagram, and represent the sensations excited in the retinal nerves by the three fundamental colours. There is some difference in opinion between physicists as to the exact wave lengths of the fundamental colours, but there is not much error in the assump tion that they are : Red—extreme end of the visible spectrum about A 665.
Green—A 507.
Blue—A 475.
The red sensation curve begins about A 680, reaches its maximum at A 575, drops strongly at A 490, with a slight rise again toward A 450, and ends at A 390. The green sensation curve begins at A 68o, rises to a maximum at A 550, and ends at A 44o. The blue sensation curve rises from 580, rises sharply at A 48o with a maximum at A 450, and ends at A 390.
The above may be considered as the now generally accepted theory of the colour sensations, but Hering suggested six fundamental colours— red, green, yellow, blue, white, and black. This theory has not been accepted, but is interesting, as on it has been based a four-colour printing method (Zander).