KNOWLEDGE AND OPINION IN REGARD TO COLOUR-BLINDNESS. 473 



2. The great body of evidence (now uncontradicted by any theoretical considerations) 

 leads to the belief that the white sensation of dichromic vision corresponds to that of 

 normal vision ; — from which it must follow that the two dichromic colours are comple- 

 mentary, not only to the dichromic, but also to the normal eye. 



3. The same evidence also tells us that the two colour- sensations correspond generally 

 with those of the normal yellow and blue. 



4. The dichromic solar spectrum consists of four divisions, which may be thus 

 described, beginning from the left hand, or long- waved end : — 



(a) Consists of the yellow colour in its full saturation, but beginning very obscure, 

 and gradually increasing to a maximum luminosity near the line D. 



(6) The yellow colour then diminishes both in luminosity and saturation to a point 

 between b and. F, called the neutral point, where the sensation becomes 

 colourless. 



(c) To the right of this point the blue colour begins, and increases gradually in 



saturation and luminosity to a maximum of both, between F and G. 



(d) From this point the blue colour, still retaining its maximum saturation, 



diminishes gradually in luminosity till it vanishes at the right-hand extremity 



of the spectrum. 

 These appearances may be explained either by intersecting yellow and blue sensation 

 curves (see Phil. Mag., July 1892), or by Herijstg's system of a single sensation element, 

 with plus and minus action, and with a separate addition of white light near the neutral. 



5. The neutral point, which divides the two colours, is at a place which to the normal 

 eye is a powerful blue-green. The complement to this, to the normal eye, is a strong 

 purple-red hue, which is not in the spectrum, but lies among the hues necessary to connect 

 the two ends, and so complete the colour-circle. These two colours, being to the normal 

 eye complementary varieties of red and green, are invisible as colours to dichromic 

 vision. 



6. Dichromic patients were formerly divided into two classes, — one class assumed to 

 be blind to the normal sensation called red ; the other, blind to the normal sensation 

 called green. This distinction is now disproved ; the patients being blind to both 

 sensations. 



7. There are, however, some considerable variations, in different patients, of the kind 

 described in Part III. But as it is now abundantly proved that variations of precisely 

 the same nature, and even to a larger extent, prevail in normal vision, their occurrence 

 also in dichromic vision becomes a natural sequel, requiring no special consideration. 



8. The only phenomena of dichromic vision undetermined are some points of detail 

 in regard to these variations. One would like to know, for example, the exact nature of 

 the connexion between the variations of the red and the green impressions ; and how 

 these variations influence the exact hues of the warm and cold colours, and the exact 

 positions of the two neutral points in the green and the red. These doubtful matters may 



VOL. XXXVII. PART II. (NO. 22). 4 C 



