64 THE VERTEBRATE RETINA 



(C) The Duplicity Theory 



History — In 1866 the great retinologist Max Schultze unobtrusively 

 announced a conclusion to which he had come after some fifteen years 

 of investigations in comparative ocular histology. He had been struck 

 by the correlation between the relative numbers of rods and cones in 

 various retinae and the habits of their possessors with regard to light. 

 Nocturnal vertebrates had many rods, and few cones or even none. 

 Diurnal species had ipany cones, and might even lack rods entirely. 

 Schultze suggested that the cone is the receptor for photopic (bright- 

 light) vision and that the rod is the organ of scotopic (dim-light) vision. 

 To this he added a corollary hypothesis that the cone alone is respon- 

 sible for color vision; for in dim light colors are no longer discriminable 

 and the world presents itself only in shades of gray. 



This theory passed unnoticed by the physiologists and early psychol- 

 ogists until, toward the end of the century, the same idea was brought 

 forward independently by two men who were led to conceive it by differ- 

 ent lines of evidence, and neither of whom knew much of Schultze's 

 work. Parinaud, studying human vision in certain pathological condi- 

 tions, produced his 'theorie des deux retines'. Von Kries, repeating and 

 extending Schultze's observations on twilight vision, with special refer- 

 ence to the vision of the retinal center, formulated the 'Duplizitats- 

 theorie' about as we have it at present. 



It is not at all uncommon for psychologists and medical men to say 

 even today that the Duphcity Theory is ^^only a theory," and to express 

 considerable doubt as to its vahdity. This ordinarily implies a con- 

 finement of knowledge to the basis of the theory in human vision. Of 

 course, if one considers only the known facts of human vision, one can- 

 not expect to be able legitimately to use very many of them to prove the 

 very theory which was evolved to explain them. But the comparative- 

 ophthalmological findings of Schultze and of many zoologists since his 

 time have built so unshakable a foundation for the theory that its major 

 tenets may be regarded as proven facts. True, there are prominent 

 French retinologists who do not believe in it, but their methods of study 

 are so antiquated that it is hardly surprising that they are unsure of the 

 distinctness of rods and cones. 



It is necessary however to bear in mind that the Duplicity Theory as 

 we state it nowadays is really two theories in one. It states that the rods 

 are responsible for the hazy, crude, achromatic (black-gray-white) per- 



