SENSATION AND SENSE-ORGANS 495 



luminosity. But, as we have already seen, no matter 

 how we stimulate the optic nerve we still get visual sensa- 

 tions; close the eyes and press with a finger-nail on one eye- 

 lid; a sensation of touch is aroused where the finger meets 

 the skin; but the pressure on the eyeball distorts it and 

 stimulates the optic nerve-fibres in it also, and the result is 

 a luminous patch seen in front of the eye in such a position 

 as a bright body must occupy in space to radiate light to the 

 stimulated part of the expansion of the optic nerve. Finding, 

 then, the same kind of sensation, a visual one, produced by 

 the totally different causes, pressure and light, we are led to 

 doubt if the differences of modality in our sensations depend 

 upon the differences of the natural forces arousing them; 

 and this doubt is strengthened when we find still other forces 

 giving rise to visual sensations. But then, since light 

 and pressure, electricity and cutting, all cause visual sensa- 

 tions, we have no valid reason for supposing that light, more 

 than either of the others, is really in any way like our sensa- 

 tion of light: or that sight-feeling differs from sound-feeling 

 because objectively light differs from sound. The eye is an 

 organ specially set apart to be excited by light, and accord- 

 ingly so fixed as to have its nerve-fibres far more often ex- 

 cited by that form of force than by any other; but the fact 

 that light sensations can be otherwise aroused shows plainly 

 that their kind or character has nothing directly to do with 

 any property of light. Just as by pinching or heating or 

 galvanizing a motor nerve we can make the muscles attached 

 to it contract, and the contraction has nothing in common 

 with the excitant, so the visual sensation, as such, is inde- 

 pendent of the stimulus arousing it and, of itself, tells us 

 nothing concerning the kind of stimulus which has operated. 

 Differences in kind between external forces being thus 

 eliminated as possible causes of the modalities of our sensa- 

 tions, we next naturally fall back upon differences in the 

 sense-organs themselves. They do undoubtedly differ both 

 in gross and microscopic structure, and the fact that pressure 

 on the closed eye arouses a touch-feeling where the skin is 

 compressed, and a sight-feeling where optic nerve-fibres are, 

 might well be due to the fact that a peripheral touch-organ 

 was different from a peripheral sight-organ, and the same 

 force might therefore produce totally different effects on 

 them and so cause different kinds of feelings. However, 



