Light and colour perception / by F.W. Edridge-Green.

  • Edridge-Green, F. W. (Frederick William), 1863-1953.
Date:
1901
Catalogue details

Licence: In copyright

Credit: Light and colour perception / by F.W. Edridge-Green. Source: Wellcome Collection.

Provider: This material has been provided by The University of Glasgow Library. The original may be consulted at The University of Glasgow Library.

    9/24 (page 309)
    colour units—that is to say, colours which can be easily dis- tinguished from each other without comparison. These approximate psycho-physical units, for a normal-sighted person, are six in number—red, orange, yellow, green, blue, and violet. Fig. 4 represents a spectrum as seen by a normal-sighted person. A and B represents the centre points of approximate psycho-physical units, green and blue; that is, colours correspond- ing to these portions of the spectrum would be easily distinguished without comparison. The rays included between A and C form an absolute psycho-physical unit; that is, the portion included be- tween A and C appears monochromatic. If shutters were used to the eyepiece of the spectroscope, the observer would not be able to say which was the yellow and which was the blue side of the portion of light shown. If, however, he were shown the coloured band from A to D, he would be able to distinguish between the colour at A and B and that at D. But if he were shown a colour corresponding to D, and were asked to which AC D B Fig. 4. portion of the spectrum it corresponded, without being allowed to compare it with other colours, he would find great difficulty in indicating the right position. There are therefore six definite points of difference in the spectrum to a normal-sighted observer, corresponding to the centre of each of the colours—red, orange, yellow, green, blue, and violet. As each of the colours at these points must, by the presence of the adjacent points, blend with each other, we have the approximate psycho-physical units formed. It will be seen that if the size of the absolute psycho-physical units, as AC, be increased, the size of the approximate units will also be increased. As the spectrum remains the same length, the increase in size of the approximate units results in their re- arrangement and diminution in number, and hence colour-blind- ness ; five instead of six distinct points are seen in the spectrum. In the next degree four, and so on, until total colour-blindness is reached. TiiEOitiES OF CoLouu Pkrception.—I will now review a certain number of the well-known facts of colour vision, and show the