16 COLOUR VISION 



and cessation of the light stimulus causes a further positive variation 

 (Holmgren), a fact of considerable theoretical interest. The electrical 

 changes have been studied by Dewar and McKendrick 1 , Kiihne and 

 Steiner 2 , Fuchs 3 , Waller 4 , Himstedt and Nagel 5 , and others, but Gotch's 

 experiments with the capillary electrometer are the most conclusive 6 . 

 He found that spectral red light gave a latent period of nearly 

 T 3 jj second and a difference of potential of 0'0004 volt ; green, T y and 

 0-0005 volt; violet, T ^y and G'00024 volt. Himstedt and Nagel 

 obtained a slight reaction with ultra-violet and Rontgen rays. They 

 also found that in the dark- adapted eye the maximum effect was 

 obtained at 544^, the site of maximum luminosity in dark adaptation 

 in man (vide infra) ; in the light-adapted eye at about the D line (589 nfi), 

 the site of maximum luminosity in light adaptation ; but though the 

 light sensibility of the dark-adapted eye is more than a thousandfold 

 that of the light-adapted for some colours, G. E. Miiller 7 obtained no 

 appreciable difference in the electrical reaction. Engelmann 8 found 

 that stimulation of one eye caused a reaction in the other also, but 

 the positive variation on removal of the stimulus was absent. The 

 relation of intensity of stimulus to strength of response does not 

 accurately follow the Weber-Fechner law (vide infra, p. 20), but 

 Talbot's law (v. p. 92) is followed more accurately (de Haas 9 ). 



When an object is looked at directly a sharp image is formed on the 

 fovea and the immediately surrounding area. An object therefore 

 which subtends less than 3 at the nodal point of the eye will form its 

 image entirely upon the rod-free area of the retina. Larger objects 

 subtending 4- -12 will form their images on the macular region, in which 

 only a few rods are present in the peripheral parts. Objects surrounding 

 that fixated form images on the peripheral regions of the retina which 

 are richly supplied with rods. The acuteness of form vision falls off 

 rapidly in passing from the fovea to the periphery, but movements of 

 objects having their images in the periphery are very readily observed. 



Not only is form vision different according as the image is at the 



? 



1 Trans. R. 8. Edin. xxvu. 141, 1873. 



- Heidelb. Unters. in. 327, 1880. 3 Arch. f. d. gcs. Physiol. LVI. 408, 1894. 



4 Phil. Trans. Roy. Soc. Lond. cxcm. 123, 1900. 



5 Ann. d. Physik, iv. 1901 ; Ber. d. Naturf. Ges. Freiburg, 1901. 



6 J. of Physiol. xxix. 388, 1903 ; xxx. 10 ; xxxi. 1, 1904. 



7 Ztsch. /. Psychol. u. Physiol. d. Sinnesorg. xiv. 329, 1897. 



8 Helmholtz' Festschrift, 197, 1891. 



9 Inaug. Dissert., Leiden, 1903; ref. in Nagel' s Jahresbericht f. Ophth., 73. 1903. 



