THE SENSE OF SIGHT. 183 



difforcnt, indeecl, a? i? tlio electric movement wliicli we excite by friction from 

 the friction itself. How tliis conversion in the retinn is brought about we know 

 not ; we can only suppose as a possibility that the waves of the ether produce, 

 ill a part of the structure of the retina, a chemical decomposition, and that 

 through this chemical decomposition a chemical irritant is generated which ex- 

 cites the current in the nerve-hbres. For this conjecture nothing further can be 

 alleged than that the undulations of light are well known to produce chemical 

 decompositions, of which the blackening of the silver combination in the sensi- 

 tive collodion plate of the photographer is an obvious example, and that certain 

 chem cal agents are capable of exciting all the nerves. Or it might be con- 

 ceived that in some Avay, perhaps, electricity is excited in the retina by means 

 of the undulations, and that this serves as an irritant for the nerves, for which 

 that agent is in general the most potent of stimulants. That electricity excites 

 the optic nerve is a well known fact ; if we pass an electric discharge through 

 the eye, a bright flash is instantly perceived. Finally, it might even be sup- 

 posed that the ether-waves produced an "expansion of some structure, and that 

 this mechanically excited th(> nerves through the consequent pressure. In short, 

 many instrumentalities are conceivable, but none is previously susceptible of 

 proof, nor can one perhaps be considered more probable than another. Only 

 one important discovery has the present age, with all its acknowledged progress, 

 disclosed to us ; we know the place of the retina in which the conversion in 

 question is effected; we know the apparatus which is charged with effecting 

 this conversion. It is not the fibres of the optic nerve running on the inner 

 sice of the retina, parallel to its surface, which are directly excited by the un- 

 dulations, though it is upon these that the undulations first strike, before they 

 adrance to other elementary portions of the membrane ; on the contrary, we 

 know with entire certainty that the waves pass, without resulting effect, not 

 only by these fibres of the optic nerve, but by the layers of the retina lying 

 r,ext behind them, until the_y have reached the external or hindermost layer 

 bordering immediately on the choroid ; their specific effect is therefore first ex- 

 erted on the so-called columnar layer with its rod-like and cone-shaped organ- 

 izations. Here the conversion takes place; the rods and cones are the apparatus 

 on which the ether-waves in some manner operate; which apparatus next 

 operates by some nukuowu agency on the nerve-fibres which bend round so as 

 tc advance to this deepest layer. It is impossible to render generally intelligible 

 tie demonstration of this statement, interesting as it is in the highest degree; 

 Ave must confine ourselves to a few cursory indications. That the fibres of the 

 o|)tic nerve in the iiitcrual layer of the retina are not directly excited by the 

 v.brations of the ethereal particles, follows necessarily from these among other 

 f{.cts — that, first, those spaces of the membrane where only nerve-fibres are 

 found, namely, at the entrance of the optic nerve, are incapable of vision, to 

 winch fact we shall recur in the sequel; and, in the second place, that at that 

 p^int of the retina with which we see most distinctly, the space just opposite 

 to the pupil, no nerve-fibres exist ; from the impossibility, moreover, of recon- 

 ciling tlie local perceptions of the organ, hereafter to be spoken of, with the 

 p-oposition that light directly excites the fibres of the optic nerve. While by 

 tiiese and other considerations it is placed beyond doubt, that light can as little 

 generate a nervous current in the fibres of the optic nerve as in any other nerve- 

 fibre, there has been direct proof adduced that it is precisely the r )ds and cones 

 before mentioned on which the light operates, that it is through these organiza- 

 tions that the excitation is effected. If, in an obscure apartment, we look fixedly 

 before us, while we move laterally in a circle before the eyes a burning candle, 

 Ave shall perceive after some time that our field of vision becomes faintly illu- 

 minated, and in this bright field a dark arborescently ramified figure makes its 

 appearance. The same phenomenon occurs if we move to and fro, against the 

 bright sky and close before the pupil of the eye, a card pierced with a small 



