182 ON THE SENSES. 



ceeds from a distant object to the eye, and is propagated througli the cornea, lens, 

 and vitreous body to the retina ? It is difficult usually for the laity, without 

 some general previous instruction in physics, to acquire a correct idea of the 

 nature of light, to disengage themselves from the deeply rooted but false notions 

 which from childhood onwards are intimately interwoven with the idea of the 

 luminous element. The sole property of light recognized by the uninformed, 

 as manifested by different colors, is no property at all of light, but, as we 

 have explained in an earlier part of these articles^is a, property of the sensation 

 which light awakens, and which has nothing iu common with the external exciting- 

 cause Wi;hout the optic nerve light is still present, but it excites no sensation, 

 imparts no color ; this paradox I hope in the following paragraph to unriddle. 

 Light is a movement, the movement, proceeding from its point of excitation, of 

 an ever present and all pervading ether, as it is called, which we must imagine to 

 be composed of countless, infinitely small, inert particles. These particles are 

 thrown into vibrations, which are propagated in the form of waves from par- 

 ticle to particle ; such is the nature of light. Luminous bodies, like the sun, con- 

 stantly set the suiTOunding particles of the ether in motion, and these propagate 

 their motions from one to another en every side. Everywhere these ethereal 

 undulations impinge upon matter ; through a part of this matter, which we term 

 transparent, they pass with more or less obstruction, the motion being transmitted 

 to the particles of the ether contained in such bodies ; through another part of 

 matter, untransparent or opaque bodies, they are unable to penetrate, and are 

 either absorbed or reflected as are the waves of sound from solid bodies, being 

 thrown back in the lalter case like the waves of the sea from its shores. If the 

 ethereal waves pass from thinner into denser matter, for instance from the air into 

 glass or Avater, or inversely from a denser into a thinner medium, they pursue 

 tlieir course undisturbed when they strike perpendicularly the surface of the 

 second medium, but, on the other hand, are diverted or refracted from their first 

 direction when they strike that surface at an oblique angle, and in a greater or 

 less degree according to the difference of density of the two mediums. A further 

 discussion of these principles of physics respecting the nature of light is here 

 impossible ; of one of the most interesting facts in this connection, the different 

 kinds, namely, of undulatory movement of the luminous ether dependent on the 

 velocity of the waves, we shall have occasion hereafter to speak more at large. 

 It is these undulations, then, of an inert, matter-penetrating ether, which, 

 proceeding from luminous bodies to our eye, throw the particles of the ether 

 already there into vibration, and are thus finally propagated to the retina. To 

 transform these vibrations into a sensation of light, no apparatus less com- 

 plicated is requisite than to convert an electrical current into a telegraphic de- 

 spatch, Avhich has as little in common with the generating current as the sensa- 

 tion of light with the exciting undulations. The sensation to which alone light 

 and color pertain as properties is a v/holly specific effect of these waves, which 

 they can only produce by help of the apparatus of the retina and optic nerve, 

 and of cerlain parts of the brain; if they strike upon wood or metal, the latter 

 become simply expanded, and this expansion bears as much relation to a sensa- 

 tion of light as a telegraphic despatch to an explosion of powder, both of which 

 may be produced by the electric current. Hence we must further inquire, what 

 becomes of the ethereal undulations in the retina; how do they operate upon the 

 elements of that membrane and the fibres of the optic nerve which' proceed from 

 it, in order finally to be converted into a sensation of light? Unfortunately very 

 little is known of the chain of processes in question ; respecting some links of 

 it we have obscure intimations; respecting others not even so much. In the 

 first place it is certain that the wave is not propagated as such in the optic nerve 

 to the brain, but that in the retina it is converted into that movement of the 

 contents of the nerve molecules which, in our introduction, we designated as a 

 nervous current, and therefore into a movement of a wholly different nature; as 



