78 LOCALIZATION OF VISUAL SENSATIONS. [BOOK m. 



other ways than by the falling of light on the retina; and the 

 point to which we wish to call attention now is that we are unable 

 to distinguish a sensation thus produced from the visual sensation 

 produced by light itself. We cannot by the help of the mere 

 sensation alone recognize the nature of the agency which has pro- 

 duced the changes in the retina giving rise to the sensation. The 

 identity of sensations due to mechanical stimulation with those 

 due to luminous stimulation may be illustrated by the story of the 

 witness in a case of assault, who swore that he recognized his 

 assailant by help of the flash of light produced by a blow on his 

 eye. Since light emitted or reflected from external objects is the 

 normal stimulus for visual sensations, all our visual sensations 

 seem to us to be produced by rays of light proceeding from 

 external objects ; we look for their cause not in the retina itself, 

 but in the external world ; and when we wish to know why we 

 have felt the sensation of a flash of light, we ignore the retina and 

 seek at once in the external world for some source of the rays of 

 light corresponding to the sensation. Hence, also, when in a 

 particular part of the retina, in a spot for instance on the nasal 

 side of the right eye, changes take place such as would be pro- 

 duced by the image of a luminous point falling on that spot, 

 though we recognize the sensation which results as having a 

 certain feature, owing to its being started in that particular spot, 

 we do not through the sensation learn anything about the retina 

 itself, we do not through it recognize that the nasal side of the 

 retina or any particular spot in the nasal side has been affected ; 

 what we do recognize, or infer, is the existence in the external 

 world of such a luminous point as would give rise to the sensation 

 in question. The dioptric arrangements of the eye are, as we 

 have seen ( 707), such that a luminous point in order to give rise 

 to an image in the spot in question, and so to the sensation in 

 question, must occupy a particular position on what we call the 

 right-hand side of the external world. We accordingly recognize 

 the sensation as having been caused by, or refer the sensation to, a 

 luminous point having that position on our right hand. And so 

 with the sensations 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 ( 670) all that our sensations can of themselves tell us 



