English: Diagram showing the Irritability of the Three Kinds of Retinal Elements: [1], red; 2, green; 3, violet. R, O, Y, G, B, V, initial letters of colours.

A theory widely accepted by physicists was first proposed by Thomas Young and afterwards revived by Helmholtz. It is based on the assumption that three kinds of nervous elements exist in the retina, the excitation of which give respectively sensations of red, green and violet. These may be regarded as fundamental sensations. Homogeneous light excites all three, but with different intensities according to the length of the wave. Thus long waves will excite most strongly fibres sensitive to red, medium waves those sensitive to green, and short waves those sensitive to violet. The figure shows graphically the irritability of the three sets of fibres. Helmholtz thus applies the theory.:—

“1. Red excites strongly the fibres sensitive to red and feebly the other two—sensation: Red.

2. Yellow excites moderately the fibres sensitive to red and green, feebly the violet—sensation: Yellow.

3. Green excites strongly the green, feebly the other two—sensation: Green.

4. Blue excites moderately the fibres sensitive to green and violet, and feebly the red—sensation: Blue.

5. Violet excites strongly the fibres sensitive to violet, and feebly the other two—sensation: Violet.

6. When the excitation is nearly equal for the three kinds of fibres, then the sensation is White.”