880 THE SENSE OF SIGHT 



Beyond the red we have rays of greater wave length, the so-called heat- 

 rays, and beyond the violet, rays of smaller wave-length, the so-called 

 chemical rays. The ultra-violet variety, however, may be raised above 

 the threshold of stimulation by rendering them fluorescent. This is 

 true of the Becquerel (radium) and Rontgen rays, the latter causing 

 a fluorescence of the retina.-^ 



It will be seen, therefore, that the sensations of color are due to 

 impacts upon the retina of ether waves of definite length, whether they 

 be derived from a homogeneous beam or from a mixture of simple lights. 

 Besides the mere color which is dependent upon the rate of vibration, 

 these sensations are also modified by the intensity or energy of the 

 vibrations as well as by the saturation of the primary color. The 

 intensity of the stimulation gives rise to luminosity or brightness. 

 Thus, it will be found that the extreme red and violet ends of the spec- 

 trum are less luminous than the yellow. Furthermore, while we are 

 able to tell which of two red or green colors is the brighter and are even 

 able to match them by increasing the intensity of the beam of light, 

 we fail absolutely when attempting to arrange different colors in strict 

 accordance with their brightness. These tests, however, may be 

 greatly varied by changing the illumination. This is shown by the 

 fact that a colored object appears colorless in low intensities of light, 

 and that the brightness of the spectrum is then shifted from the yellow 

 to the green (Purkinje's phenomenon). The saturation of a color is 

 dependent upon its admixture with white light. Thus, a perfectly 

 saturated color is one entirely free from ordinary white light, and a 

 thoroughly colored object, one which reflects specific color rays and no 

 white rays. Physically, it is not diflScult to establish this condition, 

 because all we need to do is to restrict the beam of hght to specific 

 spectral rays. Physiologically, on the other hand, color sensations 

 are generally not pure, because even monochromatic light appears to 

 give rise to sensations of white which are thus made to intermingle 

 with the particular color sensation. In other words, while the physical 

 saturation of a color may be complete,. the physiological saturation is 

 generally incomplete. 



Color Fusion. — In the same way as white light may be divided into 

 its components, so may the different spectral colors be reunited into 

 white light. This can be done very easily by placing suitable lenses 

 in the path of the colored rays emerging from a prism. It should 

 also be noted that white hght may be produced by combining only 

 two, three, four, five, or six of the original seven spectral colors. Any 

 two colors which give rise to a sensation of white are known as ^^com- 

 plementary color s.^^ 



A device most commonly employed to stimulate the retina simul- 

 taneously with two or more colors is the color-wheel of Maxwell. It 

 consists of a rotating axis to which may be attached discs of colored 



1 Birch-Hirschfeld, Archiv fur Ophthalm., Iviii, 1904, 469. 



