THE DUPLICITY THEORY 65 



cepts of dim light and that the cones yield the sharp, detailed images 

 and the chromatic (colored) sensations characteristic of bright-light 

 vision. Actually, the factors which make rod vision unsharp but sensi- 

 tive, and make cone vision sharp but requiring higher intensities of 

 illumination, are not the same as those which make rod vision achro- 

 matic and cone vision chromatic. We may be quite sure that animals with 

 rod-rich or pure-rod retinae have only diffuse mental pictures and can 

 see in very weak light, but we have at present no proof that all cones 

 are hue-discriminatory and that all rods are not. To date, no animal 

 positively known to have only rods in its retina has been properly tested 

 for color-vision capacity, and many animals which have plenty of cones 

 have been shown not to have color vision (see Chapter 12, section A). 



Sensitivity versus Acuity — When we say that an animal sees well or 

 sees poorly, that it can see in the dark or that it is blind in the daytime, 

 we are loosely jumbling together two aspects of vision which should be 

 carefully distinguished and thoroughly understood. They are indeed so 

 very different that they are practically mutually exclusive. These two 

 aspects are visual sensitivity and visual acuity. By the sensitivity of an 

 eye we mean its ability to respond to weak stimuli, the capacity it has for 

 continuing to respond to light as that light is slowly dimmed. By acuity 

 we mean the ability to continue to see separately and unblurred the 

 details of the visual object as those details are made smaller and closer 

 together. Sensitivity involves what the psychologist and physiologist call 

 'threshold of stimulation'; acuity involves what the physicist and opti- 

 cian call 'resolving power'. 



Both the sensitivity and the acuity of the vision of any vertebrate 

 depend upon the structure and mode of operation of its entire visual 

 apparatus, including the gross plan of the eyeball, the characteristics of 

 the dioptric media, the retina, the cerebral structures involved in vision, 

 and the mental capacity of the animal. But the structure of the retina 

 sets ultimate, maximal limits upon both sensitivity and acuity which can- 

 not be exceeded by any sort of manipulation of other parts of the whole 

 system. We can therefore understand these two aspects of vision well 

 enough for the time being, if we examine the retinal basis for each. 



Retinal Factors in Acuity—To consider acuity first: if the reader 

 will carefully compare a newspaper picture with one printed on the 

 glazed paper of a magazine, he will see that each is composed of dots, 

 and that the two pictures differ greatly in amount of detail. The news- 



