OHSKIiVATlONS ON VISION. 257 



wUon (ho i-(m1 ;ui(1 WIiic ends of (he spcctnmi ai'c su|)('ri)()S('(l :nul 

 obliciuoly obserNcd. 



This is otherwise shown by reconibiiiing a prismatic spectrum to 

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

 the yeUow, lirecn, and bhie-gjreen colors. Then tlie mixed Held 

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

 when obliquely observed. For a check the red or blue may alter- 

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

 is correctly seen in the renuiinino^ active color. There is a striking 

 appearance peculiar to the obliquely observed uiixed field which 

 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 light 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 Ave cease to distinguish the 

 colored papers confused by the " color-blind " only when we really 

 observe them obliquely, so that their colors are lost in gray-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 Ave no longer perceive the impres- 

 sions of the fovea. (Compare Purkinje phenomenon.) 



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

 pothesis of color-blindness color-blind persons ought to perceive 

 with direct vision also the Purkinje phenomenon and the displace- 

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

 ness. As Professor Nagel has been 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-blind persons must have lost their readiness 

 of adaptation, and therefore are equipped with a higher sensitiveness 

 for vision in strong lights than those of normal persons. 



1 Tbeso folor-iiiixing experiments were successfully performed by Professor 

 I'ringshelni. Doctor Gehrcke, and myself in a room which was lighted by two 

 incandescent lanps, although, to be sure, feebly. 



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

 then reasoning backward it follows that his fundamental experiment 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 been performed unconsciously with extra foveal — that is, 

 ■peripheral, retinal vision. 

 SM 11)04 17 



