144 COLOUR VISION 



afterwards trained by putting food in the green fork, to break the earlier 

 association and bite first at the green. 



Reighard^ fed grey snappers {Lutianus griseus) with small Atherinae 

 which were artificially coloured with seven hues. On simultaneous 

 presentation of white and blue the white was generally seized first, with 

 blue and light or dark red or yellow the blue first ; no difference was 

 apparent between blue and green. If Cassiopea tentacles were fastened 

 to the Atherinae the snappers soon learnt to avoid them. After they had 

 learnt to avoid red Atherinae with tentacles it was found that they also 

 rejected red ones without tentacles, but that they still took those stained 

 any other colour. 



The researches of Zolotnitzki, Washburn and Bentley, and Reighard 

 are consistent with the view that the fish are totally colour-blind. 



Hess examined a number of species of fish and found that they 

 appeared to behave exactly as if they had the visual perceptions of a 

 totally colour-blind man (see Part II). In the light of the spectrum 

 young Atherinae rapidly congregated in the yellow-green and green, 

 i.e., the brightest part of the human achromatic scotopic spectrum. 

 Far fewer were found in the yellow and scarcely any in the red ; there 

 were rather more in the blue. If the spectrum were displaced the fish 

 rapidly rearranged themselves in the same fashion. By moving a card 

 along the spectrum so as to intercept certain rays the fish could be 

 driven up to the blue or violet ends. If the red-yellow and green-blue 

 regions were intercepted the fish spread in both directions towards the 

 ends. If only the red and yellow-red and the blue and violet were 

 exposed many more fish collected at the violet than at the red end 

 though to the human photopic eye the red is much brighter. Just as 

 to the human achromatic scotopic eye, the red end was shortened so that 

 the extreme red produced no more effect on the fish than complete dark- 

 ness. By lighting one half of the basin with homogeneous light and the 

 other with equivalent mixed light which could be varied in intensity it 

 was possible to get the fish to arrange themselves evenly in the two 

 halves. In this manner the luminosity values of different parts of the 

 spectrum were worked out and it was found that the luminosity curve 

 agreed very well with that of the achromatic scotopic or totally colour- 

 blind human eye. Hess was also able to measure the effects of dark 

 adaptation. He found that after 15 — 20 minutes in the dark the sensi- 

 bility of the fish for light was many hundred times as great as im- 

 mediately after transference from light to dark. By using red and blue 



^ Canicgio Inst., Washington, ii. 257, l'JU8. 



