VISION OF DIFFERENT REGIONS OF THE RETINA. 1087 



a large colourless field, produced by mixture of spectral red and blue- 

 green, became red if reduced in size, and that a similar small colourless 

 field became green if increased in size. Placing a screen with two 

 apertures before such a colourless field, he found that one aperture fixed 

 directly appeared redder and darker than the other seen indirectly. 

 Hering explains the changes which take place in the relative intensity 

 of colour matches on alteration of the absolute intensity by the 

 influence of macular pigmentation (p. 1083). Breuer 1 has recently com- 

 pared colour equations for the central and paracentral regions, and found 

 that his results were in agreement with the measurements of Sachs. 



Foveal vision. — If the area of the central region used in direct 

 vision is still further reduced in size, several striking differences are 

 found to exist between its behaviour and that of the paracentral region. 

 The observation of the nature of vision with this region is impeded by 

 the great difficulty of guaranteeing accurate fixation. There seems to 

 be an instinctive tendency to look at a luminous point in such a way 

 as to see it most distinctly, and in experiments on foveal vision this 

 tendency has to be overcome. It is an old observation of astronomers 

 (Herschel, Arago) that faint stars are seen best when observed indirectly, 

 and it is now generally agreed that the threshold of stimulation for 

 light is higher for the fovea than for surrounding parts. This 

 diminished sensitiveness to light seems to extend to the whole of the 

 macula, but is much more marked in its central part. Breuer found 

 that the amount of light absorbed by the macular pigment, which 

 probably extends throughout the foveal region, was not great enough 

 to account for the loss of sensitiveness shown by the fovea. The 

 difference of sensitiveness between the fovea and surrounding part is 

 greatly increased in dark-adaptation of the eye. 



The colour vision of the fovea is approximately normal. Several 

 observers have described it as less sensitive for colour than the rest 

 of the retina, especially for blue, and Konig 2 has lately described the 

 fovea as blue-blind. It is a fact that a small blue field of low intensity 

 may become invisible on direct fixation, and this is probably due to the 

 influence of the macular pigment, which probably extends to the fovea, 

 supplemented by the low sensibility to light of that region. At ordinary 

 intensities, there is no doubt that blue is well perceived in foveal vision. 



A feature in which the fovea has, however, a distinctive character is 

 the absence or diminution of the photochromatic interval which occurs 

 for the rest of the retina. According to A. E. Fick, 3 Parinaud, 4 and 

 Konig, 5 there is no interval between the absolute and specific thresholds 

 for coloured light in the fovea, while others (Charpentier 6 and Koster 7 ) 

 find that it exists, though less marked than in indirect vision. 



A disputed point is whether Purkinje's phenomenon occurs in the fovea. In 

 addition to the ordinary difficulties, its investigation is rendered unsatisfactory 

 by the difficulty of comparison of different colours as regards brightness, 

 especially for very small areas ; but, according to Konig 8 and v. Kries, 9 

 Purkinje's phenomenon is not true of the fovea, and v. Kries notes that a small 



1 Ztschr.f. Psychol, u. Physiol, d. Sinnesorg., Hamburg u. Leipzig, 1897, Bd. xiii. S. 464. 



2 Sitzungsb. d. k. Akad. d. JVissensch., Berlin, 1894, S. 577. 



3 Arch./, d. ges. Physiol., Bonn, 1888, Bd. xliii. S. 481. 



4 Ann. d'ocul., 1894, tome cxii. p. 228. 5 Ibid. 

 " Arch, d'opht., Paris. 1896, tome xvi. ]>. 337. 



7 Arch./. Ophth., 1895, Bd. xli. Abth. 4, S. 1. 8 Loc. cit. 



9 Ibid., Bd. xlii. Abth. 3, S. 95. 



