CHAP, in.] , SIGHT. 1219 



distance apart cease to be seen as two when moved along different 

 radii from the centre, is a very irregular figure ; it differs very 

 much in different individuals ; is often not the same in the two 

 eyes of the same person, and does not necessarily correspond to 

 the figure of " the field of vision " to which we shall later on refer. 

 We may add that the power of distinguishing two points in the 

 peripheral parts of the retina is much increased by practice. 



As we have just said, when we look intently at an object such 

 as a star in the heavens we so direct the eye that the image 

 of the object falls on the fovea centralis. In the case of most 

 people, two stars so looked at appear to become one when the 

 angle subtended by the distance between them becomes less than 

 60 seconds or one minute; when they are nearer than this the two 

 sensations become one. And similar measurements are obtained 

 when other images are made to fall on the fovea, such as those of 

 parallel white streaks on a black ground or black streaks on a 

 white ground. In the case of an acute and trained observer this 

 minimum distance may be diminished to 50 seconds; in many 

 cases, on the other hand, it is not less than 73 seconds am} may 

 be more. Now the distance between two points subtended by an 

 angle of 50 seconds, corresponds in the diagrammatic eye ( 705) 

 to a distance of 3 '65 //, in the retinal image, and one of 73 seconds 

 to 5 '3 6 fi. Hence in the fovea centralis the elements of the retina 

 excited by light, must lie 3'65 fj, or 5'36 //, apart, or in round 

 numbers about 4 /JL apart, in order that the two sensations, excited 

 at the same time, may remain distinct. 



In the periphery of the retina the distance must be much 

 greater; thus at the extreme periphery, two black dots distant 

 apart about 15 mm. viewed at a distance of 20 cm. and therefore 

 giving a distance of more than a millimeter in the retinal image, 

 are still seen as one point. 



754. In accordance with the above, we may suppose the retina 

 to be divided into areas, stimulation of the retina within which 

 gives rise to a single sensation ; we might speak of these as visual 

 areas, and of the stimulation of a visual area as a sensational unit. 

 The areas are very small, and the sensational units very numerous 

 in the fovea centralis and yellow spot ; the areas are larger, and the 

 sensational units fewer, over the rest of the retina, increasingly so 

 towards the periphery. The smaller or larger the areas, the more 

 numerous or fewer the sensational units in any retina or in any 

 part of a retina, the ' more or less distinct will be the vision. 



Now in the human eye 50 cones may be counted along a line 

 of 200 fjL in length drawn through the centre of the yellow spot ; 

 this would give 4 /u, for the distance between the centres of two 

 adjoining cones in the yellow spot, the average diameter of a cone 

 at its widest part being here about 3 p and there being slight 

 intervals between neighbouring cones. Hence if we take the 

 centre of a cone as the centre of an anatomical retinal area, these 



p. 78 



