VISION OF DIFFERENT REGIONS OF THE RETINA. 1083 



with intensity. The alteration of an equation with intensity is, accord- 

 ing to Ebbinghaus 1 and Franklin, 2 shown by matching a grey, formed by 

 mixture from red and blue- green with one formed from yellow and blue. 

 If these greys are made of equal intensity with ordinary illumination, it 

 is found that, on lowering the illumination, the grey from the red and 

 blue-green becomes decidedly the brighter, and the grey derived from 

 blue and yellow requires a considerable addition of white light to enable 

 it again to match the other. According to Hering, 3 these changes with 

 intensity do not occur if the fields used are of small extent, and are 

 ascribed by him to the influence of the macular pigment ; to differences 

 in its absorption at different intensities. 4 



The experimental investigation of Grassmann's fourth proposition has 

 been rendered difficult by our want of a suitable means of determining 

 the intensities of the components in colour mixture. Eood and Abney, 

 using single direct comparison of coloured with colourless light as a test 

 of brightness, have confirmed Grassmann's fourth proposition, but the 

 subject requires reinvestigation, with the aid of more satisfactory methods 

 of colour photometry. 



When two spectral colours, which lie nearer to one another in the 

 spectrum than complementary colours, are mixed, the resultant has the 

 tone of an intermediate part of the spectrum, but is generally less 

 saturated, and usually the less saturated, the farther apart the two 

 colours. The smallest loss of saturation occurs when warm colours are 

 mixed, while the greatest loss of saturation occurs when green is one of 

 the components of the mixture. 



Another proposition, which may be added to those of Grassmann, has 

 been enunciated by v. Kries 5 and Hering. 6 Colour mixtures good for 

 one condition of retinal excitability are good for all conditions of excit- 

 ability. A match between red and blue-green on the one side, and yellow 

 and blue on the other, holds good after adaptation of the eye to red, as 

 well as in the natural condition. With marked dark-adaptation, how- 

 ever, this rule may possibly be no longer true. 



Vision of Different Eegions of the Ketina. 



Light sense of peripheral retina.— One of the factors which de- 

 termines the character of the sensation due to a certain stimulus is 

 the region of the retina stimulated. The peripheral retina will first be 

 compared with the central region taken as a whole, and then the special 

 differences will be considered which occur in the central region, char- 

 acterising the vision of the fovea and macula. Both sets of differences 

 may be considered under the three heads of light, colour, and form. 



The condition of adaptation is of the greatest importance in relation 

 to the sensibility to light. Those observers who have not paid any 

 special attention to adaptation have found a gradual decrease of sensitive- 

 ness from centre to periphery, while in dark -adaptation the threshold of 



1 Ztschr.f. Psychol, u. Physiol, d. Sinnesorg., Hamburg u. Leipzig, 1S93, Bd. v. S. 173. 



2 Proc. intemat. Cong. exp. Psych., London, 1892, p. 104. 



3 Arch./, d. ges. Physiol., Bonn, 1893, Bd. xlvii. S. 277. 



4 In a recent paper (ibid., 1898, Bd. lxx. S. 297) Tschermak, working under Hering, 

 has found changes in colourless matches with alteration of adaptation, and ascribes the 

 deviations to this factor, and not to the direct influence of altered intensity. 



5 Arch. f. Physiol., Leipzig, 1878, S. 517. 



6 Arch./, d. ges. Physiol., Bonn, 1888, Bd. xlii. S. 488. 



