THE SENSATION OF SIGHT. 207 



the nerve which belongs to it, can ever produce the sensation of 

 light. The stories of somnambiilists, which are the only argu- 

 ments that can be adduced against this belief, we may be 

 allowed to disbelieve. But, on the other hand, it is not light 

 alone which can produce the sensation of light upon the eye, 

 but also any other power which can excite the optic nerve. If 

 the weakest electrical currents are passed through the eye they 

 produce flashes of light. A blow, or even a slight pressure 

 made upon the side of the eyeball with the finger, makes an 

 impression of light in the darkest room, and, under favourable 

 circumstances, this may become intense. In these cases it is 

 important to remember that there is no objective light produced 

 in the retina, as some of the older physiologists assumed, for 

 the sensation of light may be so strong that a second observer 

 could not fail to see through the pupil the illumination of the 

 retina which would follow, if the 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 circumstances, 

 at least, there is not the smallest spark of actual light. 



In the same way, increased pressure of blood, its abnormal 

 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 pro- 

 ducing 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 external light, that persons unacquain- 

 ted with these phenomena readily suppose that the rays they 

 pee are real objective beams. 



Thus we see that external light produces no other effects in 



