80 KEGION OF DISTINCT VISION. [BOOK in. 



less than the sum of the two. A number of luminous points 

 scattered over a wide surface would appear each to have a certain 

 brightness; each would give rise to a sensation of a certain 

 intensity. If they were all gathered into one spot, that spot 

 would appear brighter than any of the previous points ; the 

 intensity of the sensation would be greater. 



753. The region of distinct vision. The distance at which 

 two images must be apart from each other in order that the two 

 sensations may be separate is not the same for the whole area of 

 the retina. If two luminous points lie near the optic axis, so that 

 their images fall on the fovea centralis or on the yellow spot, they 

 will be seen as two distinct points, even when their images lie very 

 close indeed to each other. If the luminous points be moved 

 aside, so that the images fall on the retina outside the yellow spot, 

 the two luminous points, though at the same distance apart from 

 each other, will give rise to one sensation only, and be seen as 

 one point ; they may be moved even farther apart from each other 

 and still give rise to one sensation ; and if the two points be placed 

 so much on one side that their respective images fall on the 

 extreme peripheral parts of the retina near the ora serrata, the 

 two images may be separated from each other a very considerable 

 distance and yet give rise to one sensation only. We may vary 

 the experiment by making use of a negative sensation, and take 

 two black dots on a white surface only just so far apart that they 

 can be seen distinctly as two when placed near the axis of vision 

 so that their images fall on or near the fovea, and then, keeping 

 the axis fixed, move the two points outwards, so that their images 

 travel outwards from the fovea ; it will be found that the two 

 soon appear as one. The two sensations become fused, as they 

 would do if brought nearer to each other in the centre of the 

 field. The farther 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 ' 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 meridians. The outline 

 described by a line uniting the points where two spots at a certain 



