Chap, hi.] SIGHT. 885 



which are distributed to the optic nerve as nervi nervorum. Such 

 experiences are urged in support of the view that all impulses 

 passing along the optic nerve however generated, whether by 

 retinal changes or by other means, are visual impulses and visual 

 impulses only ; they give rise to visual sensations and to visual 

 sensations alone. On the other hand, in other cases of removal 

 of the eye in the absence of anesthetics, neither section of the 

 optic nerve nor subsequent stimulation of the stump has given 

 rise to visual sensations. We shall return to this question later 

 on when we have to speak of what is known as the " specific 

 energy of nerves," and have only referred to it incidentally now. 

 § 554. Visual sensations then may be produced in many 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, in the dark, 

 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 produced by the image of a luminous point fall- 

 ing 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 exist- 

 ence in the external world of such a luminous point as would 

 give rise to the sensation in question. The dioptric arrange- 

 ments of the eye are, as we have seen (§ 529), such that a lumi- 

 nous 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 hav- 

 ing been caused by, or refer the sensation to, a luminous point 

 having that position on our right hand. And so with the sensa- 



