THE SENSATION OF SIGHT. 235 



sensation were really produced by an actual development 

 of light within the eye. But nothing of the sort has 

 ever been seen. Pressure or the electric current excites 

 the optic nerve, and therefore, according to Miiller's 

 law, a sensation of light results, but under these cir- 

 cumstances, at least, there is not the smallest spark of 

 actual light. 



In the same way, increased pressure of blood, its ab- 

 normal constitution in fevers, or its contamination with 

 intoxicating or narcotic drugs, can produce sensations of 

 light to which no actual light corresponds. Even in 

 cases in which an eye is entirely lost by accident or by 

 an operation, the irritation of the stump of the optic 

 nerve while it is healing is capable of producing similar 

 subjective effects. It follows from these facts that the 

 peculiarity in kind which distinguishes the sensation of 

 light from all others does not depend upon any peculiar 

 qualities of light itself. Every action which is capable 

 of exciting the optic nerve is capable of producing the 

 impression of light ; and the purely subjective sensation 

 thus produced is so precisely similar to that caused by ex- 

 ternal light, that persons unacquainted with these pheno- 

 mena readily suppose that the rays they see are real ob- 

 jective beams. 



Thus we see that external light produces no other 

 effects in the optic nerve than other agents of an entirely 

 different nature. In one respect only does light differ 

 from the other causes which are capable of exciting this 

 nerve : namely, that the retina, being placed at the back 

 of the firm globe of the eye, and further protected by 

 the bony orbit, is almost entirely withdrawn from other 

 exciting agents, and is thus only exceptionally affected 

 by them, while it is continually receiving the rays of 

 light which stream in upon it through the transparent 

 media of the eye. 



