1052 VISION. 



purple. The times taken by the various phases have been studied by 

 Fuchs ; he found a latent period of -0004 to -0061 sec. ; a duration of 

 the positive phase varying from -007 to "0181 sec. ; the time to the end 

 of the negative phase varied from '0089 to "0352 sec. 



Engelmann and v. Grijns x found that light applied to one eye of a 

 frog caused, after a short latent period, a marked variation of the current 

 in the other eye, the change being an increase in the negativity of the 

 equator in relation to the cornea. The same result is obtained with 

 chemical stimulation of one eye. Conduction by the glandular skin of 

 the head was excluded. The influence of other movements was also 

 excluded by the use of curari, which was found not to prevent the 

 movement of cones and pigment. 



The effect of coloured light on the electrical variation is a point of 

 very great importance which has not yet been satisfactorily investigated. 

 Dewar and M'Kendrick 2 found that yellow light produced the largest 

 variation, green next, and then red and blue. These and similar results 

 obtained by other observers are probably dependent on the different 

 intensities of the coloured lights. All colours appear to give a variation 

 in the same direction. 



Dewar and M'Kendrick 3 found that the relation between luminosity 

 of the stimulus and electrical variation was approximately the same as 

 that existing between stimulus and sensation. When the luminosity 

 increased from one to a hundred units, the variation was multiplied 

 from three to six times. Waller 4 has investigated the relation more 

 thoroughly, and has found a logarithmic relation between stimulus and 

 variation. 



Visual Sensations. 



Introductory. — The facts of light and colour vision will now be 

 considered, avoiding, as far as possible, reference to the physiological 

 structures and processes concerned, and only those general and theo- 

 retical questions will be discussed which do not involve any special 

 theory of vision. After the facts have been described, an account 

 will be given of the known relations between visual sensations and 

 retinal elements, and of the chief physiological theories of vision. 



Visual sensations may be divided into three groups, namely, sensations 

 of light, of colour, and of form. The perception of form is closely 

 associated with other kinds of spatial perception, in which binocular 

 vision and movements of the eyes are the chief physiological factors, 

 and will be more fully dealt with in the last part of this article. In 

 this part it will only be considered in so far as it seems to be a function 

 of the retina without movement, as in the discrimination of two points. 

 The sensibility for light or luminosity is very closely associated with that 

 for colour, but the two will be considered separately as far as possible. 



The character of a sensation which follows stimulation of the retina 

 always depends on two factors, — the nature of the stimulus, and the 

 condition of the part of the retina stimulated. Each of these factors 

 is complex, and consequently the character of the sensation may be 

 influenced by many circumstances, including the nature of previous 

 stimulation of the retina ; the size and position of the area stimulated ; 

 the duration of the stimulus, and the rate at which individual stimuli 



1 "Beitr. z. Psych, u. Phys. Sinn.," 1891, S. 197. 



2 Trans. Boy. Soc. Edin., 1876, vol. xxvii. p. 141. 3 Ibid., p. 156. 



4 Brain, London, 1895, vol. xviii. p. 200 ; also more recently in Broc. Boy. Soe. London, 1900. 



