118 COLOUR VISION 



Charpentier^ published a series of experiments which confirm and 

 extend those of Ricco. He used small bright squares up to 12 mm., 

 viewed at 20 cm. distance. Below 2 mm. the smaller the surface the 

 greater the minimum illumination necessary for perception. Two 

 millimetres at this distance correspond to about 0"17 mm. on the retina, 

 i.e., about the size of the fovea. For larger areas the area has no effect. 

 It follows, therefore, that in order to produce a luminous sensation at 

 the fovea the total quantity of light, i.e., the product of the area and 

 the illumination, must attain a certain value, and that that value is 

 constant for a given condition of adaptation. " The fovea centralis 

 forms a sort of autonomous territory, in which the luminous excitation 

 diffuses itself, and which always requires a certain quantity Of light 

 to be set in activity." Charpentier showed, in answer to criticisms by 

 Leroy, that the diffusion could not be accounted for by irradiation due 

 to dioptric aberrations. 



Asher^ found that for the range of light intensities used by him up 

 to a visual angle of 2' to 3' the apparent size depends entirely upon the 

 quantity of the light. According to him, therefore, vision of objects 

 subtending angles up to this size is a function purely of the light sense 

 and not of the form sense. The earliest observations bearing upon this 

 aspect of the subject were by Volkmann (1863) and Aubert (1865). 

 Aubert used lines 2 mm. wide and 50 mm. long, and determined the 

 distance the lines had to be apart in order that the interspace might 

 look the same as the breadth of the lines. He found that when the 

 breadth of the lines was varied by Volkmann's macroscope so that they 

 subtended visual angles of from 10" to 45" the angular distance apart 

 of the lines varied from 104" to 112" for black lines on a white background, 

 and from 140" to 153" for white lines on a black background. Asher 

 used small black, white, and grey squares and rectangles of paper and 

 determined the distance at which a difference of size could be detected, 

 the light intensities of the papers being calculated by the colour top 

 and by Hering's polarisation photometer. He found that visual angles 

 between 23" and 78" might be increased by 25" to 100" (average 58") 

 under the given differences of light intensity (from 360 to 6) before a 

 difference in size could be detected. At a great distance from the 

 different sized objects they appear either of equal size and brightness, 

 or, if the objective brightness of the smaller is much greater than that 

 of the larger, of equal size but unequal brightness, the smaller being the 



1 Compt. rend. xci. 1880 ; Arch. d'Ophf. n. 234, 487, 1882. 



2 Ztsch.f. Biol. XXXV. 394, 1897. 



