OBSEKVATIONS ON VISION. 255 



r»ut what aiiatoniiciil ix-ciiliarity have the colics which (Miahles 

 them to (listiii'iuiish'ivd from bhie ? For it seems ch'ar that this dis- 

 tinction is made in tire retina and not in the brain. 



It is a fact that in the eyes of l)irds. which, moreover, have many 

 more coik^s tlian rods over the whole extent of the retina, all the 

 cones ha\'e color-ditferentiatin<i' organs; oil cells colored red, yellow, 

 blue, and ii'reen. V>\\t these oil cells are found only in the cones of 

 birds and certain ami)hibioiis animals and rej^tiles, not at all in man. 

 And yet our cones have a special structure ditferent from the rods, 

 for the outer members show a series of phine |)arallel i)lates whose 

 distance apart Max Schultze estimates to be in conformity with 

 the wave lengths of light. May not these plates serve to dilferen- 

 tiate the light as in Lij)])man's method of color photography? 

 Against this view, which I tiiid was jiropounded a long time ago 

 by AV. Zenker" and Max Schultze. and then again apparently for- 

 gotti'ii. there is tlie objection that the rods also in " hardening" show 

 a distinct j)latelike se])aratioii. But l)efore using the fact to o))pose 

 (he hypothesis above mentioned, it must be proved that the rods show- 

 ing this ])latelike sei)aration are not from the edge of the macula 

 liitca. I am inclined to believe that the rods which border u]:)oii 

 ihe macula hitea tend to approach a more conelike lyi)e than th-.' 

 others. 



However this may be, the i)hcii()inen()ii of total color blindness or 

 lack of all color pei'ce|)tion can scarcely l)e satisfactorily explained 

 by the '^'oung-IIelmliolt;'; or the Ilering theories, which assume that 

 I he AMsual sul)stanccs are either fused into one or partly absent. 

 A(hiiitting, however, that the distinction of colors belongs to the cones 

 and colorless vision to the rods, it is easy on the other hand to explain 

 total color-I)lindness by the hy[)othesis that in such cases the cones are 

 absent. This explanation was advanced by If. Kiinig 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 have been desigimted by Max Schultze as '' rod seers." The in- 

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

 I'ods alone are pres(>nt and are extremely line (thickness 0.75 /<), 

 whil(> the existence of a few cones is only known by the occurrence 

 of cone grannies in the outei' granular layer. 



'* Rod seers " then \iew 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 jx'rsons in darkness. Totally color-blind 

 persons see the sun-lighted landscape as a i-od landscape, as we see it 

 by faint moonlight. Total color-blindiu^ss is therefore a very marked 

 defect. Some persons are. howe\'er, partialh' color-blind, by which is 



a W. Zenker, I.ehrbuch der Pliotochrouiie, Braunschweig, Fr. Vieweg u. Sohti, 

 J 900. 



