48 Light and the Eye /2 : 4 



curves can be interpreted as an indication that luminosity depends on at 

 least two types of receptors. The simplest interpretation might be to 

 assign scotopic vision to the rods and photopic vision to the cones. 

 This choice would be indicated by the fact that the scotopic sensitivity is 

 greater in the periphery where there are more rods, whereas the photopic 

 response is greatest in the fovea where there are no rods. However, this 

 separation of function is definitely oversimplified; the rods appear to be 

 active in both dark-adapted and light-adapted eyes, whereas the cones 

 are active only in light-adapted eyes. 



C. Acuity 



Studies of the acuity of vision also indicate that the rods are the active 

 elements in the dark-adapted eye. The acuity of the eye adapted to 

 scotopic vision is a minimum at the fovea where there are no rods. 

 Thus, the rods seem to be the active elements in scotopic vision. The 

 acuity of scotopic vision shows a maximum for light at the retinal region 

 where the rod density is highest, namely, about 20° from the fovea. 

 Acuity under scotopic conditions is lower than under photopic con- 

 ditions in any region of the retina. The neural basis for this is discussed 

 in Chapter 7. However, in photopic vision there is a sharp maximum 

 in the ability to resolve two spots of light when the images fall on the 

 fovea. The acuity in the foveal region is much greater than in the 

 remainder of the retina. 



The acuity of vision may be expressed in terms of the minimum angular 

 separation of two equidistant points of light which can just be resolved. 

 The angular separation 6 between two points, when expressed in radians, 

 is approximately equal to the distance between the points divided by the 

 distance from the eye, provided 6 is less than 0.1. The angle 6 will 

 also be equal to the separation of the two images on the retina divided 

 by the distance from the second nodal point. From Equation 4, one 

 can calculate a minimum value of 6, according to the Rayleigh criterion, 

 for green light (A — 5 x 10 ~ 5 cm) and an iris diameter of about 0.5 cm. 

 Rounding off to one significant figure, the limit, according to this 

 criterion, would be 



d R == I x 10~ 4 radians == 0.03 minutes of arc 



This is a theoretical lower limit for the resolution of two points of light. 

 Experiments have shown that most people cannot resolve two points 

 of light if their separation is as small as 5 x 10 ~ 4 radians. Persons with 

 the most acute vision can resolve an angular separation of about 

 2 x 10 ~ 4 radians under optimum conditions. Because this is higher 

 than the Rayleigh criterion, it seems that visual resolution must be 



