1096 VISION. 



especially in lowered conditions of nutrition, and has also been observed 

 in connection with diseases of the liver (Parinaud). 1 There seems to be 

 little doubt that the essential feature of the condition is abnormal slow- 

 ness of dark -adaptation. It has been found that a night-blind individual 

 may attain the same degree of sensitiveness at low intensities as normal, 

 but requires for it a much longer period of adaptation. 



Schirmer 2 objects to the view that night-blindness depends simply on 

 adaptation of abnormal slowness, on the ground that, immediately after 

 going into feeble illumination, the night-blind see much worse than 

 normal individuals. This may, however, be connected with the momen- 

 tary adaptation described by Hering (p. 1080). Treitel 3 found that the 

 bright-adapted eye, when placed suddenly in feeble illumination, behaved 

 in many ways like the night-blind eye. 



Photophobia, such as that which occurs in albinos, may be regarded 

 as an abnormal condition of the upper limit of adaptation. It does not 

 disappear with a small artificial pupil. Schirmer 4 has found that in this 

 condition the threshold is normal, and dark-adaptation takes place 

 normally in both rate and extent. 



Subjective colour sensations. — The most frequent of these is red 

 vision, or erythropsia. It has frequently been described as occurring 

 after removal of the lens for cataract and after strong stimulation, such 

 as that of light reflected from snow. It has been observed in conjunction 

 with night-blindness. It has been produced experimentally by Fuchs 5 

 by exposure to snow-light with an artificially dilated pupil. The white 

 squares of a black and white chessboard pattern appeared red, while the 

 black squares were greenish. The condition has been referred to entoptic 

 vision of visual purple, but no satisfactory explanation is yet forthcoming. 



Cases of other kinds of subjective colour-vision have been described, 

 such as green vision (Somya), 6 either affecting the whole visual field or 

 in the form of coloured globes. Blue vision often occurs after removal 

 of the lens for cataract. It is connected with the yellow coloration of 

 the lens. 



Intrinsic light (of the retina?). — This increases in intensity with 

 dark-adaptation of the eye, and the variations in its appearance have 

 been described by Aubert. 7 Aubert regarded it as probably of central 

 origin, and the same view has been advocated by others. Pilehne 8 

 found that the intrinsic light was continuous over the central scotoma of 

 the visual field which existed in his own eye, and since his defect was 

 probably due to affection of the nervous path, rather than of the retina, he 

 held this to point to the central origin of the intrinsic light. It has been 

 suggested (Dufour) 9 that absence of vision and cloudy vision (p. 1095) 

 may be characteristic of cortical and subcortical hemianopia respectively ; 

 and if found to be so, the fact would point to the central origin of the 

 intrinsic light, but the clinical evidence on this point is at present 

 scanty and conflicting. 



1 Arch. gen. de mid., Paris, 1881, Stk. 7, tome vii. p. 403. 



2 Verhandl. d. X. internat. mod. Cong., Berlin, 1890, Bd. iv. Abtb. 10, S. 58. 

 a Arch. f. Ophth., 1887, Bd. xxxiii. Abth. 4, S. 73. 



4 Verhandl. d. X. internat. med. Cong., Berlin, 1890, Bd. iv. Abth. 10, S. 58. 



5 Arch./. Ophth., 1896, Bd. xlii. Abth. 4, S. 207. 



6 Ztschr. f. Psychol, u. Physiol, d. Sinnesorg., Hamburg u. Leipzig, 1894, Bd. vii. S. 305. 



7 " Physiol, d. Netzhaut," S. 333. 



8 Arch./. Ophth., 1885, Bd. xxxi. Abth. 2, S. 1. 



9 Rev. med. de la Suisse Pom., Geneve, 1889, tome ix. p. 445. 



