ON LIGHT. 359 



tympanum of the ear which receives the impulse of the 

 aerial medium, would appear to vibrate like the parch- 

 ment of a drum, by the direct impact of its waves per- 

 pendicular to its surface. It is, therefore, sensible to 

 such of the movements of the vibrating medium only as 

 are in the direction of the sound-ray, and not at all to 

 transverse vibrations. But if we conceive the nervous 

 filaments of the retina as minute elastic fibres, standing 

 forth at right angles to its plane, like the bristles of a 

 brush (the reader will pardon the apparent coarseness ot 

 the illustration, which is only intended as an illustration 

 of what may be, and no doubt is, a process of trans- 

 cendent delicacy), immersed in the ether ; it is evident 

 that movements of the latter parallel to their direction 

 would not, but that those transverse to it would tend to 

 throw them into vibration, just as eare of corn would be 

 little or not at all agitated by a straight ^and slender 

 rod moved up and down between the stalks, or to and 

 fro in the direction of its own length, but violently by 

 a transverse horizontal motion of the rod. 



(140.) Whatever be, at any instant, the motion of an 

 ethereal molecule, it may always be resolved into two, 

 one in the direction of the ray in the act of propagation, 

 and the other in a direction transverse to it, in the plane 

 of the wave surface. If the sensation of light be sup- 

 posed to be produced by the former resolved portion, 

 no account can be given of the phenomenon of polar- 

 ization ; such movement being equally related to sur- 

 rounding space in all directions outward from the ray 

 as an axis. The contrary is obviously the case with 



