1086 VISION. 



limit for yellow about 1° smaller than that for blue, and refers this to 

 the difference of dispersion of the two kinds of light. 



Guillery x has recently investigated the colour-sense of the peripheral 

 retina, by using as his measure the extent of the retinal image which 

 enables the colour to be recognised. He found for each colour gradual 

 increase necessary from centre to periphery. The figures for yellow and 

 blue showed a close agreement, but differed considerably from those for 

 red and green, which corresponded well with each other, thus confirming 

 the result of Hess by an entirely different method. It may be 

 mentioned here that the photochromatic interval is found to be 

 greater for the peripheral than for the central retina, and that Hess 

 found that a colour equation good for the central (extramacular) region 

 also held good for the periphery. 



Visual acuity of peripheral retina. —The difference between central 

 and peripheral retina in respect of visual acuity, is far greater than 

 for other kinds of retinal sensibility. It has been tested by finding 

 the distance at which two points can be discriminated, and also by 

 means of letters. Groenouw 2 and Guillery 3 have also used lately the 

 size of the " physiological point " as a test. Acuity is most intense in 

 direct vision, and drops very rapidly at first, and then more slowly 

 towards the periphery. The low degree of acuity in the. periphery can 

 be greatly improved by practice (Dobrowolsky and Gaine 4 ), differing in 

 this respect from the sensibility for light and colour. The diminished 

 acuity has been referred to four factors, — anatomical differences, want 

 of practice, greater spherical aberration, and diminished intensity in 

 indirect vision. It is probable that the first three of these causes all 

 take part in the production of the difference. Koester 5 has found that, 

 with dark-adaptation, visual acuity is equally good in all parts of the 

 retina, except in the fovea where it is lowered ; while in light-adapta- 

 tion there is an enormous increase for central vision. 



Macular vision.— So far, in comparing the peripheral with the 

 central retina, it has been assumed that the central area has uniform 

 sensibility. This is very far from being the case. The macula, and still 

 more the fovea, differ in their behaviour both to light and colour 

 stimuli from that part of the central region around them which for 

 distinction may be called the " paracentral " region. 



The influence of the macula is chiefly due to its yellow pigmentation, 

 and shows itself chiefly in colour mixture, the relative intensity of 

 different parts of the spectrum being altered owing to the absorptive 

 action being greater for some colours than for others. The relative 

 degree in which the different parts of the spectrum are absorbed by the 

 pigment of the extirpated retina was studied by Sachs. 6 Hering had 

 previously found no absorption of red, and Sachs found the absorption 

 still very slight at D, with a slight increase between D and E, which 

 became more rapid between E and F, reaching a maximum near F, 

 then falling off slightly to the end of the violet. The influence of the 

 macula on colour matches has been studied by Hering. 7 He found that 



1 Ztschr.f. Psychol, u. Physiol, d. Sinnesorg., Hamburg u. Leipzig, 1896, Bd. xii. S. 267. 

 - " Ueber die Sehschiirfe d. Netzbautperipherie," Wiesbaden, 1892. 



3 Ztschr. f. Psychol, u. Physiol, d. Sinnesorg., Hamburg u. Leipzig, 1896, Bd. xii. S. 243. 



4 Arch.f. d. ges. Physiol., Bonn, 1876, Bd. xii. S. 411. 



5 Centralbl.f. Physiol., Leipzig u. Wien, 1896, Bd. x. S. 433. 



6 Arch.f. d. ges. Physiol., Bonn, 1891, Bd. 1. S. 574. 



7 Ibid., 1893, Bd. liv. S. 277. 



