THE SENSATION OF SIGHT. 233 



sense of taste, if the wire was laid upon the tongue. Or, 

 again, a strong electric current may be transmitted by 

 telegraphic wires in order to ignite gunpowder for blasting 

 rocks. In short, everyone of the hundred different actions 

 which electricity is capable of producing may be called 

 forth by a telegraphic wire laid to whatever spot we 

 please, and it is always the same process in the wire itself 

 which leads to these diverse consequences. Nerve-fibres 

 and telegraphic wires are equally striking examples to 

 illustrate the doctrine that the same causes may, under 

 different conditions, produce different results. However 

 commonplace this may now sound, mankind had to work 

 long and hard before it was understood, and before this 

 doctrine replaced the belief previously held in the constant 

 and exact correspondence between cause and effect. And 

 we can scarcely say that the truth is even yet universally 

 recognised, since in our present subject its consequences 

 have been till lately disputed. 



Therefore, as motor nerves, when irritated, produce 

 movement, because they are connected with muscles, 

 and glandular nerves secretion, because they lead to 

 glands, so do sensitive nerves, when they are irritated, 

 produce sensation, because they are connected with sensi- 

 tive organs. But we have very different kinds of sensa- 

 tion. In the first place, the impressions derived from 

 external objects fall into five groups, entirely distinct 

 from each other. These correspond to the five senses, and 

 ' their difference is so great that it is not possible to com- 

 pare in quality a sensation of light with one of sound 

 or of smell. We will name this difference, so much 

 deeper than that between comparable qualities, a differ- 

 ence of the mode, or kind, of sensation, and will describe 

 the differences between impressions belonging to the same 

 sense (for example, the difference between the various 

 sensations of colour) as a difference of quality. 



