SECTION VI 



AREAL EFFECTS. 

 CHAPTER I 



THE LOCAL QUANTITATIVE EFFECT 



Some effects of the size of the area stimulated have already been 

 touched upon {v. pp. 51, 79, 95). They are of considerable importance 

 and demand more detailed consideration. 



With regard to foveal vision it may be said that given a sufficiently 

 intense illumination for a given condition of adaptation a mathematical 

 point of light will be visible. Its image on the retina is always a diffusion 

 area, the central parts of which afford at least a minimal effective 

 stimulus if they anywhere impinge upon a retinal cone. If the effective 

 area is so small as to occupy an interconal space a slight movement of 

 the eye must be predicated in order that the point may be visible. It is 

 probable that for a given condition of retinal adaptation a subminimal 

 stimulus for a single cone may become a minimal or eft'ective stimulus 

 if spread over several cones. 



Ricco^ conducted a very careful series of experiments bearing upon 

 this point. The experiments were carried out in six different ways^. 

 He found that at the threshold of sensibility the quantity of light 

 entering the eye is constant, or, in other words, the light intensity and 

 the area of the retinal image are reciprocal functions, or the product 

 of the area into the light intensity is constant. In terms of the visual 

 angle, the law is that the minimum visual angle varies inversely as the 

 square root of the light intensity, or the product of the minimum visual 

 angle and the square root of the light intensity is constant. The limit 

 of the law is determined by the size of the foveal region ; it ceases to be 

 accurate for visual angles above 40' to 50'. 



1 Ann. di Ottal. vi. 1877. 



2 Parsons, Roy. Lond. Ophth. Hasp. Rep. xix. 1, 114, 1913. 



