SIGHT AND THE VISUAL ORGAN. 461 



tion originates is endowed with specific sensory action, so the irritation 

 of that part can alone produce impressions of vision ; hut this irritation 

 may he imparted to it by other parts of the brain, or by other nerves. 

 It has been already stated that irritations of the brain produced by 

 narcotics are transmitted only by proximity to the terminal extremity 

 of the optic nerve. At the same time, it may be that the irritation 

 proceeds from another nerve, from a nerve of touch or of hearing, and, 

 penetrating to the brain, affects it so strongly as to send on the con- 

 cussion to the optic centre. It is this that takes place when, after 

 having listened to disagreeable sounds, you are seized with certain sen- 

 sations in the nerves of touch, for instance, in those of the teeth ; or, 

 heving gazed into the bright light, you become aware of a tickling 

 sensation in the nose, causing you to sneeze. In a word, it is here a 

 question of so-called sympathy, to be explained by transmission of the 

 irritation from one nerve to the other. 



The disposition to such sympathetic sensation is increased by a 

 general irritability of the nervous system; while, in a calmer state of 

 the nerves, the excitations run more regularly in the paths directly 

 affected by the originating causes. In this manner, those indirectly- 

 provoked visual impressions which preponderate in circumstances of 

 sickness and disease are augmented. 



In these indirect visual sensations, as in the direct excitation of 

 the mechanism of the visual nerve, only subjective sight has been 

 treated of, cut off from from every relation with the outer world. We 

 are quite ready to attach credit to the fact that, at exhibitions of som- 

 nambulism, when the natural irritability of nervously-disposed individ- 

 uals is heightened, subjective visual impressions are produced in an 

 unusual degree. Should, however, any connection with surrounding 

 objects be founded on these results, or any transmission of the specific 

 sensory action into other parts, as, for instance, transmitting to the 

 skin of the abdomen the power of producing objective visual percep- 

 tions, such as are necessary for reading, these assurances are to be 

 ranged in the same category of physiological blunders as the Munchau- 

 sen hunting-story. 



By what means, then, does the mechanism of the visualnerve, which 

 we have hitherto regarded merely as the instrument of subjective sight, 

 become a practical bridge between our ideas and the outer world, and a 

 medium of the true and accredited operation of the senses f I answer, 

 by a normal relation to a definite irritation proceeding from an object. 

 This, which we might call the adequate sensory irritation, is light. 



Let us consider the general relation between light and the organ 

 of sight. Unable to discover with certainty the nature of light, it is 

 explained in physics as being the undulating motion of an elastic body 

 called ether, diffused throughout the universe. According to this, the 

 irritation by light represents the shock of the undulations of ether on 



