Page:Popular Science Monthly Volume 1.djvu/472

458 down postulates on the nature of the soul. But, since the beginning of the world, this manner of committing our thoughts to the guidance of metaphysical hypotheses has never increased our knowledge 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 human 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 science, 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 organ which, from the enormous amount of material it is the means of bringing to the mind, takes a prominent place in the part assigned 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 irritation 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 irritation was conducted to the brain, and called forth this sensation in that