45 8 THE POPULAR SCIENCE MONTHLY. 



ing down postulates on the nature of the soul. But, since tbe begin- 

 ning of the world, this manner of committing our thoughts to the 

 guidance of metaphysical hypotheses has never increased our knowl- 

 edge even by a hair's breadth. 



Fortunately, the majority of thinkers have now struck into the 

 more promising paths of observation and analysis. Essential and 

 most important progress has been made in the knowledge of the hu- 

 man body, since men have ceased to indulge in subtle speculation into 

 the nature of the principle of life, and turned with an undivided spirit 

 of inquiry into the laws of organic appearances. And since men have 

 applied themselves to trace with care the psychical manifestations of 

 perception into the world of ideas, and to do the utmost in their power 

 to discover the laws which there govern, there has arisen another sci- 

 ence, forcing itself more and more on our notice as it daily proves 

 itself to be possessed of an inherent vitality. I refer to the science of 

 psychology. 



Such being our starting-point, the operations of the perceptions 

 acquire a wider significance. Resting on this increased significance, I 

 now venture to bring before you the structure and functions of that or- 

 gan which, from the enormous amount of material it is the means of 

 bringing to the mind, takes a prominent place in the part as- 

 signed to the operation of the senses. If I succeed in heightening a 

 little your interest in this organ, or even only in reanimating in some 

 of you the sentiment of happiness which must fill every grateful child 

 of the Creator, when, on awaking in the morning, he joyfully greets the 

 light of day, I shall have earned a rich reward for a trifling exertion. 



Suppose, as shown in Fig. 1, that the brain which reposes in the 

 cavity of the cranium, and is the bodily organ of consciousness, runs 

 off, at one spot of its complicated structure, into a cord-like process, 

 which lengthens till it reaches the surface of our body, when it then 

 spreads out again in an umbellar form. Imagine, further, this whole 

 process, including its roots, endowed with a specific sensitiveness, by 

 virtue of which it responds with a luminous sensation to every irrita- 

 tion applied, and you have a fundamental idea of the nervous part of 

 the visual organ. 



Before proceeding further, let us first become better acquainted 

 with these parts. A, indicates the brain ; B, the aforesaid process, or 

 in other words the optic nerve, which, passing through an aperture in 

 the cranium, stretches on till it reaches into the orbit, where it spreads 

 out into that expanse C, which, under the well-known term of retina, 

 turns its surface to the outer world. Lastly, X is the point where the 

 process is inserted in the brain, the letter X meaning to indicate the 

 still unknown extent of its connections. 



When I said, above, that every point of the whole mechanism, on 

 being irritated, produced a luminous sensation, I meant that the irrita- 

 tion was conducted to the brain, and called forth this sensation in that 



