20 EEPOKT— 1893. 



intensity of the light is gradually diminished. As the colours fade away 

 they become indistinguishable as such, the last seen being the primary 

 red and green. Finally they also disappear, but a grey band of light 

 still remains, of which the most luminous part is that which before was 

 green.' Without entering into details, let us consider what this tells us 

 of the specific energy of the visual apparatus. Whether or not the 

 faculty by which we see grey in the dark is one which we possess in 

 common with animals of imperfectly developed vision, there seems little 

 doubt that there are individuals of our own species who, in the fullest 

 sense of the expression, have no eye for colour ; in whom all colour sense 

 is absent ; persons who inhabit a world of grey, seeing all things as they 

 might have done had they and their ancestors always lived nocturnal 

 lives. In the theory of colour vision, as it is commonly stated, no reference 

 is made to such a faculty as we are now discussing. 



Professor Hering, whose observations as to the diminished spectrum 

 I referred to just now, who was among the first to subject the vision of 

 the totally colour-blind to accurate examination, is of opinion, on that 

 and on other grounds, that the sensation of light and shade is a specific 

 faculty. Very recently the same view has been advocated on a wide basis 

 by a distingaished psychologist, Professor Ebbinghaus.'^ Happily, as 

 regaids the actual experimental results relating to both these main 

 subjects, there seems to be a complete coincidence of observation between 

 observers who interpret them differently. Thus the recent elaborate 

 investigations of Captain Abney ^ (with General Festing), representing 

 graphically the results of his measurements of the subjective values of 

 the difi'erent parts of the diminished spectrum, as well as those of the 

 fully illuminated spectrum as seen by the totally colour-blind, are in the 

 closest accord with the observations of Hering, and have, moreover, been 

 substantially confirmed in both points by the measurements of Dr. Konig 

 in Helmholtz' laboratory at Berlin.'* That observers of such eminence 

 as the three persons whom I have mentioned, employing different methods 

 and with a different purpose in view, and without reference to each 

 other's work, should arrive in so complicated an inquiry at coincident 

 results, augurs well for the speedy settlement of this long-debated 

 question. At present the inference seems to be that such a specific 

 energy as Hering's theory of vision postulates actually exists, and that 

 it has for associates the colour-perceiving activities of the visual appa- 

 ratus, provided that these are present ; but that whenever the intensity 



1 Hering, ' Dntersuch. eines total Farbenblinden,' PJiuger's Arch., vol. xlix., 1891, 

 p. 5(53. 



2 Ebbinghaus, ' Theorie des Farbensehens,' Zeitschr.f. Psychol., vol. v., 1893, p. 145. 

 ' Abnej and Festing, Colour Photometry, Part III. Phil. Trans., vol. clxxxiiiA, 



1891. p. 531. 



* Kinig, ' Ueber den Helligkeitswerth der Spectralfarben bei verschiedener 

 absobiter Intensitiit,' Beitrdge zur Psychologic, &c., ' Festschrift zu H. von Helmholtz,' 

 Siebzigsten Geburtstage,' 1891, p. 309. 



