888 REGION OF DISTINCT VISION. [BOOK m. 



away from the centre of the field, the farther apart must two 

 points be in order they may be seen as two. 



It is obvious that the more sharply we can distinguish the 

 several sensations produced by the images of the several points 

 of which any external object may be supposed to be made up, 

 the more distinct will be our vision of the object. In the fovea 

 centralis our power of thus distinguishing sensations is at its 

 maximum ; in the outer parts of the yellow spot around the 

 fovea it is less ; just outside the yellow spot it is much less ; 

 and thence diminishes more gradually towards the periphery of 

 the retina. Hence we speak of the fovea ' centralis, including 

 more or less of the whole yellow spot, as the "region of distinct 

 vision ; " and when we wish to examine closely the features of 

 an external object, we so direct the eye, we so 4 look ' at the 

 object, that its image falls as far as possible on the fovea 

 centralis. The diminution of distinctness does not take place 

 equally from the centre to the circumference along all meri- 

 dians. The outline described by a line uniting the points 

 where two spots at a certain 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 maybe diminished to 50 seconds; 

 in many cases, on the other hand, it is not less than 73 seconds 

 and may be more. Now the distance between two points sub- 

 tended by an angle of 50 seconds, corresponds in the diagram- 

 matic eye ( 527) to a distance of 3-65 //, in the retinal image, 

 and of 73 seconds to 5-36 /*. Hence in the fovea centralis the 

 elements of the retina excited by light, must lie 3-65 //, or 5-36 p 

 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 there- 



