362 PHYSIOLOGY CHAP. 



not equally distinct ; the portion lying between Fraunhofer's D 

 and E lines, between orange and yellow, stands out by its extreme 

 brightness ; passing from this region towards the red end or the 

 violet end the brightness diminishes at first rapidly, then more 

 slowly, then rapidly again. This difference in the luminosity of 

 the colours of the spectrum is not due to differences in the energy 

 of the ether vibrations of different wave-length, for we know that 

 the thermal effects of the red rays, which are less bright, are 

 greater than those of the yellow rays, which are brightest, and 

 that maximal heat effects are obtained from the ultra-red rays, 

 which are invisible. The difference in brightness depends, there- 

 fore, not 011 a different energy of vibration, nor on any objective 

 difference in the respective monochromatic lights, but on the 

 internal constitution of the receptor elements of the retina. The 

 yellow rays are the brightest of all, because the receptors are 

 more sensitive to these than to the other rays. 



When the illumination is weak and the eyes are well adapted to 

 darkness, the sensibility to rays of different wave-lengths is much 

 altered. According to Hering and Hildebrand (1889), the spectrum 

 then appears entirely colourless, and the relative brightness of the 

 different bands is quite different from that in the strongly illumin- 

 ated spectrum. While in the latter the maximal brightness lies 

 between yellow and orange, that is, towards the less refrangible 

 bands, and the minimal brightness in the more refrangible bands, 

 in the weakly illuminated spectrum the maximal brightness lies 

 between green and blue, that is, towards the more refrangible 

 bands (which appear brighter than any others, in spite of their 

 want of colour), and the minimal brightness lies in the red, the 

 least refrangible band, which is sometimes completely invisible. 



The colour sensibility of the light-adapted eye is not equal 

 over the whole of the retina ; it is greatest at the fovea, and 

 decreases thence to the ora serrata. The most peripheral parts 

 are quite colour-blind, and are only capable, therefore, of giving 

 colourless sensations (Fig. 174). Between the fovea, in which 

 sensibility to colour is maximal, and the peripheral parts of the 

 retina, in which there is complete colour-blindness, lies an inter- 

 mediate zone which is blind to red and green. In this area red 

 and green lights appear to be a more or less saturated yellow, 

 gradations of yellow being plainly visible, though this colour 

 cannot be distinguished from either red or green. It is, however, 

 to be noted that the inner and outer limits of this red-green 

 blind area are only relative, and alter with the experimental con- 

 ditions. When the size, photic intensity and chromatic satura- 

 tion of the object are increased, these limits recede more or less 

 towards the periphery of the retina. In fact, when an object is 

 fixated by the intermediate area of the retina, and its size or 

 luminous intensity is gradually increased from that at which it 



