352 ANNUAL OF SCIENTIFIC DISCOVERY. 



Turning from the sense of hearing to that of sight, a precisely simi- 

 lar analysis holds good. Here the vibrating medium is not the at- 

 mosphere, but a universally diffused ether, which is set in motion by 

 what are called luminous bodies. Just as atmospheric oscillations 

 form the external cause, and sound the internal result, in the case 

 of hearing, so, in sight, the oscillations of the light-bearing ether form 

 the outward condition, and color, in all its various shades, the inward 

 result. Here, accordingly, as before, it is simply motion in nature 

 giving rise to motion in the nerve-world, with which we have imme- 

 diately to do in vision ; while, to keep up the analogy, it is the differ- 

 ence in the rapidity of the oscillations that creates all the infinite 

 variations of hue. The red rays, it is calculated, require four hun- 

 dred and fifty-eight billions of oscillations in a second, the violet rays 

 seven hundred and twenty-seven billions, and all the other colors 

 and shades of the spectrum some intermediate number. That the 

 phenomena of sound and sight spring physiologically out of particular 

 states of the corresponding nerves is clear from the fact that pres- 

 sure on the eye, or any artificial irritation, produces the perception 

 of light as strongly as the normal impulses derived from the vibrating 

 ether, and that any artificial excitement of the auditory nerve will 

 produce noise in the head. Ghost-seeing often arises in the same 

 way ; that is, when the conditions of sight are brought about by the 

 nerves being affected through some other than the ordinary and 

 legitimate stimuli. Whatever, in a word, can affect the regular vital 

 movements of the nerves, and put them into a condition at all similar 

 to that produced by the proper external stimuli of sensation, will of 

 necessity bring about similar phenomena of consciousness. 



We come next to the sense of feeling. This sense comprehends 

 two apparently distinct series of sensations, namely, those of touch, 

 properly so called, and those of heat. With regard to the latter, it 

 has been pretty well established that the phenomena of heat originate 

 in the oscillations of a subtle fluid similar to that of light. The sen- 

 sation of heat may, therefore, be brought under the law of motion just 

 as much as that of light or hearing, and may be regarded in every 

 respect as analogous. The phenomena of touch, we know, are pro- 

 duced by impact in various ways ; and it is just in accordance with 

 the nature of that impact, whether harder or softer, more rapid or 

 more slow, that the resulting sensations are determined. A blow 

 is a sudden affection produced by the rapid motion of some object 

 against a considerable surface of the body. Pressure is a more con- 

 tinuous affection of the same kind. A prick is the motion of some 

 object against one minute point of the skin. If the act of pricking be 

 repeated rapidly, it produces a feeling of burning, and, if it be very 

 soft at the same time, of itching. An extremely light and gentle 

 motion over the body produces tickling. In every instance the pecu- 

 liar kind of sensation is determined by the nature of the motion and 

 the consequent impact. 



The only two senses left, accordingly, are those of taste and smell. 

 In both these cases the process by which the nerves are affected is of 

 a chemical nature. The substances received upon the surface of the 

 tongue or the internal membrane of the nostril are subjected to the 

 action of saliva or mucus, and, being thus dissolved, produce a chemi- 



