622 A TEXTBOOK OF PHYSIOLOGY 



and surrounding objects are localized and identified. The sensitive- 

 ness of the retina has increased many times. Conversely, on emerging 

 from darkness to light, one is " dazzled," owing to this extreme 

 sensitiveness. This soon passes off, and the eye becomes " light- 

 adapted." 



Experiment seems to indicate that the different regions of the 

 retina have different functions. The peripheral regions are found 

 to be relatively more sensitive than the fovea to feeble stimuli that 

 is, to light of moderate or short wave-lengths. On the other hand, 

 the central portion of the retina responds particularly to bright light 

 light of long wave-lengths. When the eye becomes adapted to 

 the dark " dark-adapted " the increased responsiveness of the 

 retina under these circumstances is in the regions outside the fovea. 

 It is much easier to perceive stars of small magnitude when looking 

 sideways than when looking directly at them. It is for this reason, 

 also, that a star may be suddenly observed in the heavens during a 

 movement of the head, and yet, when that part of the heavens is 

 scanned directly, it cannot be seen. 



On the other hand, as already stated, it is known that it is in the 

 fovea centralis that vision, especially form sensation, is most acute. 

 We always look directly at a thing when we want to appreciate its 

 shape. At the same time, the pupil is contracted to shut out peripheral 

 rays. In order to differentiate similar objects grouped closely together, 

 it is necessary that these should subtend an angle of a certain magni- 

 tude at the nodal point with respect to the eye. Further, in order that 

 objects majr be differentiated, it is necessary that their contiguous 

 margins and the space between should form an image on the retina, 

 which should riot be less than a certain length. It has been found 

 that a subtended angle of 63-75 seconds, equivalent to a retinal 

 distance of 0-00463 millimetre, is necessary for discrimination. Double 

 stars, which subtend an angle less than this, appear to the naked eye 

 as single stars. 



The acutenos.s of vision at the fovea is ordinarily tested by noting 

 the distance at which letters, which at a given distance subtend an 

 angle of 5 minutes at the eye, can be read. This method may be 

 applied either to ascertain what error of refraction may exist in the 

 eye, or, if this be absent or corrected, what the acuteness of vision 

 in the particular eye is. We shall see later that it is also in the region 

 of the fovea that the different colours are best appreciated. 



The so-called duplicity theory supposes that there are two distinct 

 visual mechanisms in the retina one that of the rods and visual 

 purple, upon which depends achromatic reactions, especially under 

 conditions of darkness adaptation; secondly, that of the cones, which 

 serves achromatic responsiveness in bright light, and also chromatic 

 responsiveness. This view is not altogether accepted, but it is sup- 

 ported by the fact that a great abundance of rods and visual purple 

 is found in animals which see badly in broad daylight, but which 

 have good " twilight vision." Such animals are the bat, owl, and 

 hedgehog. Cones, on the other hand, predominate in the retinae of 



