FUNCTIONS OF THE RE TINAL STR UCTURES. 1 1 o i 



to 147 '5". With fine black wires against the sky, separated from each 

 other by a distance corresponding to the diameter of the wires, Helmholtz 

 found for his own eye an angle of 63""75 necessary for discrimination. 

 This corresponds in Listing's eye to a retinal distance of -00463 mm. 

 The thickness of the cones in the macula has been determined by several 

 observers, the figures given varying from 

 •0015 (H. Miiller) to -0054 (Kolliker). 

 In making his observations, Helmholtz 

 noticed at a certain distance the appear- 

 ances shown in Fig. 395, A. He explained 

 the appearance by the arrangement of the 

 cones, as is shown in Fig. 395, B. 



From the figures above obtained Helm- 

 holtz calculated the number of cones which 

 should be found on a square millimetre, 

 namely, 13,466 ; and this agrees very closely with the number of cones in 

 a square millimetre of the fovea as counted by Sulzer, 1 about 13,500. 

 Observations obtained by du Bois-Keymond 2 also agree very closely. 

 This observer determined the distance at which a number of (460) holes 

 in a sheet of metal were discriminated when looked at against the sky, 

 and found that in different observers the points were perceived when 

 they ranged from 7400 to 14,900 per square millimetre of the retinal 

 surface. These observations, together with the nature of the vision of 

 the peripheral retina, make it most probable that visual acuity is 

 dependent on the cones, i.e. at ordinary luminosities. 



Function of visual purple. — Till comparatively recently, no syste- 

 matic attempt had been made to bring visual sensations into relation 

 with any known retinal substances. When visual purple was discovered, 

 it was hoped that it would become the basis of the physiological theory of 

 vision, but its absence from the place of most distinct vision was so great 

 a difficulty that it was soon neglected as an important factor in vision. 



In 1867, however, before visual purple was discovered, the occurrence 

 of rods alone in nocturnal animals had led Max Schultze 3 to the con- 

 clusion that rods were in some way concerned with vision at low intensities. 

 The chemical behaviour and great instability of visual purple suggested 

 the same idea to Kiihne. 4 Parinaud 5 was led by the consideration of 

 pathological night-blindness to regard visual purple as the basis of vision 

 at low intensities, and found this confirmed by the nature of foveal vision. 

 Charpentier 6 held at one time similar views. More recently the same idea 

 has been much more fully developed, especially by v. Kries, 7 who appears to 

 have independently reached conclusions identical with those of Parinaud. 



According to this view, the great increase of sensitiveness to light 

 of the dark-adapted eye is due to accumulation of visual purple. The 

 fact that adaptation to brightness takes place with much greater 

 rapidity than to darkness, agrees with the fact that visual purple is 

 bleached so much more rapidly than it is regenerated ; and there is, 

 further, a rough correspondence between the rates of the two processes, 



1 Sitzitngsb. d. k. Akad. d. Wisscnsch., Wien, 1880, Bd. lxxxi. S. 7. 



2 Arch.f. Ophth., 1886, Bd. xxxii. Abth. 3, S. 1. 



3 Arch.f. mikr. Anat., Bonn, 1866^ Bd. ii. S. 175. 



4 Journ. Physiol., Cambridge and London, 1878, vol. i. p. 189. 



5 Compt. rend. Acad. d. sc, Paris, 1881, tome xciii. p. 286 ; and 1885, tome ci. p. 821. 



6 Jbid., 1878, tome Ixxxvi. p. 1341. 



7 Ztschr.f. Psychol, u. Physiol, d. Sinnesorg., Hamburg u. Leipzig, 1895, Bd. ix. S. 81. 



