OBSKRVATIONS ON VISION. li^f) 



r»ul what aiiiitoniical |>cciiliarily have the roiics which (Mial»h^> 

 them to (lisliiiiiuish red from hhic^ Foi- it scciiis clear that this dis- 

 tinction is made in the retina and not in the hrain. 



It is a fact that in tlie eyes of hirds. which, moi-eo\-ei-, liave many 

 more cones than rods over tlie whole extent of the retina, all the 

 cones lunc coloi--(lill'erentiatin<2: oi'<>ans: oil cells colored red. yellow, 

 hhie. and <j;i'een. I'lit these oil cells ai'e found oidy in the cones of 

 hirds and certain amphibious animals and reptiles, not at all in man.- 

 And \et our cones have a special structui'c diU'erent from the I'ods, 

 for the outer meuihers show a series of plane parallel plates whose 

 distance apart Max Schultze estimates to be in conformity with 

 tlu' wave len«^ths of lioht. May not these plates serve to diii'ei-en- 

 tiate the li<2:ht as in Lij^pmanV method of color photo<rraphy ? 

 Apiinst this view, which 1 find was jjropounded a lonij time ago 

 by "\V. Zenker" and Max Schultze, and then again apparently for- 

 eotteii. ther(> is the objection that the rods also in '* hardening " show' 

 a distinct platelike separation. But l)ef()re using the fact to o])pose 

 the hypothesis above mentioned, it must be proved that the rods sliow- 

 ing this platelike separation are not from the edge of the macula 

 lutea. I am inclined to believe that the rods which border ui)on 

 the macula lutea tend to approach a more conelike type than the 

 others. 



However this may be, the phenomenon of total color blindness or 

 lack of all color perce])tion can scarcely be satisfactorily explained 

 l)v the Young-llehnholtz or the Ilering theories, wdiich assume that 

 the visual substances are either fused into one or partly absent. 

 Admitting, however, that the distinction of colors Ixdongs to the cones 

 and colorless vision to the rods, it is easy on the other hand to exi)lain 

 total color-blindness l)y the hypothesis that in such cases the cones are 

 absent. This explanation was advanced by IT. K(»nig and supported 

 by the fact that all animals which live in darkness (the bat, mole, 

 hedgehog, nocturnal monkey, and others) have no cones. These ani- 

 mals hav<' been designated by ^Slax Schultze as " rod seers." The in- 

 vestigations of (ireeff show that in the fovea centralis of the rat also 

 i-ods alone are present and are extremely line (thickness 0.75 //), 

 while tlie existence of a few cones is only known by the occurrence 

 of cone granules in the outer gianular layer. 



'• Rod seers '' then view everything, even in broad daylight, as color- 

 less, and, in case their fovea centralis has no rods, are, at this spot, 

 totally blind, like ordinary persons in darkness. Totally color-blind 

 jiersons see the sun-lighted landscape as a rod landscape, as we see it 

 by faint moonlight. Total color-blindness is therefore a very marked 

 defect. Some persons ar(>. howevei'. partially color-blind. Iiy which is 



." W. Zenlcer, Lehrbuch der Pljotochroiuie, Braunschweig, Fr. Viovvog u. Solin. 

 I'JOO, 



