886 LOCALIZATION OF VISUAL SENSATIONS. [Book hi. 



tions similarly generated in all other spots in the retina ; we 

 recognize them as caused by luminous points occupying such 

 positions in the external world that their images fall on those 

 spots. In each case we ignore the retina itself, and the changes 

 taking place in it are to us simple tokens of luminous events in 

 the external world. When with the right eye we see a luminous 

 point on our right-hand side, if we know that changes are taking- 

 place on the nasal side of the retina of that eye, it is not because 

 we are directly aware that the nasal part of the retina is being 

 affected, but because our knowledge of the dioptrics of the eye 

 teaches us that the image of the luminous point is falling on the 

 nasal side of the retina. If we are suffering from right-sided 

 hemiopia (§ 497) all that our sensations can of themselves tell us 

 is that we cannot see things on the right-hand side ; they do not 

 tell us anj^thing about the retina itself ; they cannot even tell us 

 whether the deficiency of vision is due to changes failing to be 

 set up in the retina or to the cerebral centres failing to be affected 

 by the retinal changes ; such questions we have to decide by 

 some other means than a simple examination of our sensations, 

 and by a similar roundabout way only are we able to conclude 

 that in such a hemiopia it is the nasal side of the right retina, 

 and the temporal side of the left retina, which fail to give rise to 

 visual sensations. Our sensations, in fact, tell us of themselves 

 nothing about the optical image on the retina ; they do not tell 

 us whether the retinal image is inverted or no ; the fact that the 

 retinal image is an inverted one does not in itself influence our 

 visual sensations, and hence the inversion needs no correction on 

 our part. 



§ 555. As we have just said, if the images of two luminous 

 objects, two luminous points, fall on the retina at a certain dis- 

 tance apart, the consequent sensations are distinct. If, however, 

 the two objects are made to approach each other, a point will be 

 reached at which the two sensations are fused into one. Two 

 stars at a certain distance apart may be seen distinctly as two 

 stars, while two stars nearer each other appear to be one star ; 

 we cannot analyze the latter sensation into its constituent 

 parts. 



Similarly, when the images of a number of luminous points, 

 of equal luminosity, fall on the retina sufficiently near each 

 other, the effect is not a number of sensations of luminous 

 points, but one sensation, that of a luminous surface. This 

 introduces a new feature of visual sensations, namely, that of 

 size. If the luminous points be few, so as to involve a small 

 area of the retina, the sensation is that of a small surface ; if 

 the luminous points, equally near to each other as before, be 

 numerous, so as to involve a large area of the retina, the sensa- 

 tion is that of a large surface. Moreover, such a sensation of a 

 surface will be referred to some position in space corresponding, 



