OBSEKVATIONS ON VISION. 257 



wlion the rod aiul Itlue ends of the si)ectriiin aiv suporjiosod and 

 obli<iueIy ohsorved. • 



This is otherwise shown by recombinino- a prismatic spectrum to 

 produce white, but with the interposition of a screen which cuts off 

 the yellow, green, and bhie-o-reen coh)rs. Then the mixed Held 

 appears of a brilliant rose-red by direct \ision, but a brilliant white 

 when obliquely ol)ser\'ed. For a check the red or bhie may alter- 

 nately be cut off, and then by oblique vision the apparently white field 

 is correctly seen in the remaining active color. There is a striking 

 appearance peculiar to the obliquely observed mixed field Avhich 

 gives one the impression that it is self-luminous or fluorescent." 



It is hard to say why it is that color-confusing persons see red 

 and green pigment fields as gray- white and confuse them, while per- 

 sons of normal vision sharply distinguish between them. A possible 

 explanation might be that in bright liglit the fovea centralis is the 

 principal agent of vision overcoming the peripheral retinal field, 

 and that we see extensive bright fields not at one time but by sweep- 

 ing over them with the fovea. Thus we cease to distinguish the 

 colored papers confused l)v the "" coloi--blind "" only when we really 

 observe them obliquely, so that their colors are lost in gi*ay-white, 

 even in daylight. In the condition of adaptation to obscurity, on 

 the contrary, the fovea centralis is excelled by the peripheral region 

 of the retina rich in rods, so that we no longer perceive the impres- 

 sions of the fovea. (Compare Purkinje phenomenon.) 



A greater difficulty seems to me to be tliat according to this hy- 

 l)othesis of color-blindness color-blind persons ought to 2>ei'ceive 

 with direct vision also the Purkinje phenomenon and the displace- 

 ment of the "neutral zone'' which follows diminution of bright- 

 ness. As Professor Nagel has l)een good enough to inform me, how- 

 ever, the Purkinje phenomenon absolutely ceases with him, at least 

 at the fovea centralis.'- Since, however, his fovea centralis is color- 

 blind, I find myself compelled to add a second hypothesis, as follows: 



The rods which occur in the fovea centralis and part of those in 

 the macula lutea of color-l)lind persons must have lost their readiness 

 of adaptation, and therefore are etjuipped with a higher sensitiveness 

 for vision in strong lights than those of normal persons. 



a These color-mixing experiments were successfnlly performed by Pi'ofessor 

 Pringsheim, Doctor Gehrclvo, and myself in a room wliieli was liglited by two 

 incandescent lanps. although, to be sure, feebly. 



& If this is so with Professor Brodhun (who confuses red and green) also, 

 then reasoning backward it follows that his fundamental exiieriment on the 

 displacement of the neutral zone (a distinct proof of the competition of rods 

 and cones) owing to the smallness of the field of the Ilelmholtz color-mixing 

 apparatus, must have l)een i)erformed unconsciously with extra foveal — that is, 

 periphci'al, retinal vision. 

 SM 1!:KH 17 



