THE LOCAL QUANTITATIVE EFFECT 121 



Conversely, diminution of the area stimulated causes the colour to 

 lose in apparent intensity red less than green, green less than blue. 

 The law associating area stimulated and intensity is therefore not so 

 simple as that for white light. 



Abney 1 in his experiments on the extinction of colour and light, 

 i.e., on the point in the diminution of the intensity of light which just 

 causes, first the colour, and then the light to become invisible, made a 

 series of investigations on the influence of the area stimulated. He 

 found, as was to be expected from the results of previous observers, 

 that the smaller the disc the less reduction in intensity of the ray was required 

 to extinguish it and the same ratio existed betiveen the extinction of the 

 different colours. Plotting curves with aperture diameters in powers 

 of 2 as abscissae and logarithms of light intensities as ordinates, with 

 apertures less than 1| inches diameter the curves become straight lines, 

 all of which are parallel. Hence " from that point the intensity of a 

 light which will be just extinguished with a certain diameter of aperture 

 may be increased 10 times and yet be invisible when an aperture with 

 one quarter of that diameter is employed ; if the intensity of the light- 

 be increased 100 times, we have only to diminish the diameter of the 

 aperture to j 1 ^ and it will again disappear, or if to 7 ^, the light may be 

 increased 1000 times." When the angular aperture exceeds 4 apparently 

 the upper limit is reached, all extinctions being the same beyond it. 



With regard to the point of extinction, Abney 2 says : ' ' The light 

 from a square, or a disc, or an oblong, just before extinction, is a 

 fuzzy patch of grey, and appears finally to depart almost as a point. 

 This can scarcely account for the smallest width of an illuminated 

 surface determining the intensity of the light just not visible ; but it 

 tells us that the light is still exercising some kind of stimulus on the 

 visual apparatus, even when all sensation of light is gone from the outer 

 portions. The fact that the disappearance of the image takes place 

 in the same manner whether viewed centrally or excentrically tells us 

 that this has nothing to do with the yellow spot, or fovea, but is probably 

 due to a radiation of sensation (if it may be so called) in every direction 

 on the retinal surface. Supposing some part of the stimulus impressed 

 on one retinal element did radiate in all directions over the surface of the 

 retina, the effect would be greatest in the immediate neighbourhood, 

 and would be inappreciable at a small distance, but the influence 

 exerted upon an adjacent element might depend not only on its distance, 

 but also upon whether it was or was not itself excited independently. 

 1 Abney, pp. 169, 174. 2 Ibid. p. 177. 



