182 SATURDAY LECTURES. 



sense of vision resides in the cortical substance of the pos- 

 terior cerebral lobes. 



But the human mind, ever unsatisfied with its present 

 knowledge, inquires still further, and asks what are the 

 alterations which take place in that particular portion of 

 the brain when the changes Avrotight by the light waves on 

 the retina are brought to it through the medium of the 

 optic nerve? 



Let us, for a moment, look at it dispassionately and with- 

 out prejudice, as becomes all seekers after the truth. Let us 

 remember in the first place that we have been all the while 

 dealing with matter. The object to be seen is material, the 

 ether, if it exists, must be material ; and we know that the 

 optic nerve and retina are matter, and the brain is a sub- 

 stance analogous to these in anatomical structure. 



If we take a self-luminous body as the object, we know 

 that molecular change, in the form of motion, forms the 

 first link in the chain of sequences of which we spoke at 

 the outset of this lecture as necessary for the function of 

 vision. These motions are communicated to the surround- 

 ing ether, and by it are carried to the retina. Have we any 

 right to suppose that the molecular motion stops there? 

 Certainly not. There is no fact yet demonstrated which 

 militates against it, and in this hand to hand warfare of 

 Science against the hidden secrets of Nature we claim all' 

 to be for us which is not positively against us. On this 

 principle, therefore, we are amply justified in assuming that 

 the ether has only been the medium of transference of mol- 

 ecular motion from the body to the retina. This motion is 

 then transferred through the optic nerve to the demonstrated 

 center of vision in the brain. Does molecular change in 

 the form of motion end even then? Having followed it 

 thus far we can see no satisfying reason why it should. 

 And the last link in the chain which is discernable by our 

 short-sighted vision is a change, manifested by a form of 

 motion in the ultimate molecules of the brain. How this 

 motion of the molecules is converted into consciousness is a 

 problem for the solution of which we have not even the 



