254 OBSERVATIONS ON VISION. 



VII. OCILAI} riIA> TOMS. 



The hallucinntioiis of rod vision are best illustrated when a small 

 surface is observed Avhose retinal iinag-e does not exceed the area of 

 most distinct vision and whose gradual increase of brightness can be 

 noted, beginning with darkness. I employ the following api)aratus: 

 An electrical glow lamp of (;r)-volt type is placed in a box with a sin- 

 gle aperture IH mm. in diameter, and connected to a 110-A'olt circuit 

 whose current may be gradually varied from zero upward. Behind 

 the opening is placed a dark shutter, and in front of this a movable 

 brass diaphragni with different holes of 3, 6, and 9 nnn. diameters. 

 An enlarged, distinct image of the sharply bounded aperture is then 

 cast on a white screen, and there are placed in the jjath of the beam 

 several sheets of gelatin stained blue-green, so that the rods are more 

 strongly imi^ressed than the cones. 



So long as the brightness is so feeble that the cones are excluded, 

 the light spot, seen oblicpiely, appears colorless and lacking sharp 

 outlines; but it quite disappears when gazed at fixedly. This dis- 

 appearance occurs even Avhen the light spot is so large that its image 

 covei-s the Avhole of the macula lutea, as shown by the introduction 

 of the larger diaphragm. To be sure, the disappearance is more 

 difficult to produce, because the slightest movement of the eyes then 

 renders the spot visil)le, and the Avandering eye involuntarily takes the 

 position where it receives the most light. Hence it hapjoens that the 

 spot floats in and out of the vision, for as soon as we attempt to fix 

 distinctly what we saw indirectly the "ghost" disappears." But 

 when the liglit is made so bright that the cones are im})ressed this 

 condition of things ceases, and Ave see the light spot distinctly and at 

 rest Avith its shai-p outlines and its blue-green color. The experiment 

 is les« successful Avhen performed Avith red stained gelatin, but even 

 in this case the spot ma}' be caused to disapi)ear by proper reduction 

 of the illumination, as in P]x])eriment III. where the dark-red gloAv- 

 ing lamp filament becomes invisible by direct vision, Avhile yielding 

 the gray-glow ajjpearance with oblicjue vision. 



1). Kxi'EiiiMKN rs wrni I)i{i(;irr Spkc ri;.\. 



I I lyiHitlicsis MS to lilt' c.-uisc (if (•()l(H--l)liiHlin'ss. I 



Discrimination between the rods and cones and acipiaintance Avitli 

 their different fmictions has not yet furnished the answer to th(^ ([ues- 

 lion why the cones differentiate colors, or Avliat means they emplov. 

 and why the i-ods grow more sensitive in obscurity. It is true that 

 the cones are destitute of the visual purple, so that perhaps this plays 

 a part in " adaptation to obscurity." 



"Sec (). F.iiiMiiicr. licit r.ii,' ymv Kliiruii.i,' del- ni'ucstcii \(M-sii(1i(' voii R. Blondlot 

 iiber die u-rays. Vcrh. dt-r Deutscbon Phys. (ioscllsclmft, .1. 418, inO.S. 



