be regarded as the result of refinements of common sensation-each a more refined touch. Coarse vibrations are perceived by the nerves of common sensation as a jarring. When the vibrations are so rapid that

Unmodified

Cellular Structure

DIAGRAM ILLUSTRATING THE LAW OF DIFFERENTIATION.

there are sixteen complete movements back and forth in a second, an entirely different sensation is produced, which we call sound. The vibrations are no longer perceived by the nerves of common sensation, but a

special nerve-the auditive-is organized to respond to or co-vibrate with them. As the vibrations increase in number, they are perceived as higher and higher pitch, until they reach the number of about 30,000 to 40,000 in a second. This is the highest pitch the ear can perceive, the quickest vibrations the auditive nerve can respond to. Beyond this there is no sensation of any kind, but only because we have no nerve organized to co-vibrate with these more rapid undulations. These vibrations, insensible to us, may possibly be perceived by some lower animals, as, for example, insects; we can not tell. After a long interval, vibrations again appear in consciousness as light. The vibrations which produce this sensation are so rapid—the lowest about 400,000,000,000,000 in a second-that they can be conveyed only by the ethereal medium. For the perception of these vibrations, a peculiar and wonderful organization is necessary, found only in the optic nerve. Above the number just given, ethereal vibrations are perceived as different colors, in the order seen in the spectrum, until about 800,000,000,000,000 is reached.* Beyond this

we have no nerve capable of responding.

2. As to Kind of Contact.-The gradation among the special senses may be shown in a different way. In touch we require direct and usually solid contact; in taste, liquid contact, for uniess a body is soluble it can not be tasted; in smell, the contact is gaseous, for un

*It is difficult to conceive the extreme rapidity of the vibration of a ray of light. The following illustration may help: The average or green ray vibrates about 600,000,000,000,000 times in a second. If t represents the time of one vibration, then t: 1 scc. : : 1 sec. :: 20,000,000 years-i. e., the time of vibration of a ray of light is the same part of one second as one second is of twenty millions of years, or there are as many vibrations in one second as there are seconds in twenty millions of years.

less a body is volatile or vaporizable it can not be smelled. In this last case, the perception of objects at a distance begins; still it is by direct contact, for particles from the distant body must touch the olfactive nerve. Thus far the impression is immediate. In hearing, there is no direct contact of the sounding body, but the vibrations are conveyed through a medium. We perceive at a distance, limited only by the extent of the atmosphere and the energy of the initial vibration. This sense is therefore still terrestrial. In sight, finally, we perceive objects at a distance which is illimitable, the vibrations being conveyed by a medium which is universal, and too subtile to be recognized except as the bearer of light. This sense, therefore, is cosmical.

3. As to Objectiveness.-Again, commencing with taste: In this sense we distinctly perceive that the sensation is subjective—is in us, not in the body tasted. In smell, there is an equal commingling of subjectiveness and objectiveness. We distinctly perceive the sensation as in the nose, and yet by experience we have learned to refer it to an object at a distance. In hearing, we already refer the cause so completely to a distant object that there is but the smallest possible remnant of a consciousness of sensation in the ear; the sound does not seem to be in the ear, but in yonder bell. Finally, in sight, the impression is so completely projected outward-seems so absolutely objective—and the consciousness of anything taking place in the eye so completely lost, that it is only by careful analyses that we can be convinced of its essential subjectiveness.

Distinction of Higher and Lower Senses.-The order which we have given above is also the order of increasing specialization and refinement of the senses.

But only in the two higher senses-only in those senses in which there is no direct contact, but the impressing force is conveyed by means of vibration through a medium-only in these highest senses do we find that, besides the specialization of the nerve-fibers to respond to peculiar vibrations, there is also an elaborate instrument placed in front of the specialized nerve in order to intensify the impression and give it more definiteness. It is wholly by virtue of this supplementary instrument that we are able to hear not only sound but music, or to see not only light but objects. The lowest animals in which an optic nerve is found perceive light, but not objects; because, though the specialized nerve is present, the appropriate instrument is wanting. Thus hearing and sight are widely different from the other senses. and stand on a higher plane. It is on these two higher senses that fine art is wholly, and science is mainly, founded. The specialized nerve and the instrument for intensifying and making definite the impression are together called the sense-organ. It is of the most highly specialized of these nerves and the most refined of these instruments, the highest of the sense-organs, the eye, that we are now about to treat.

It may be well to bear in mind and keep distinct what may be called the direct gifts of sight, and what are added by the mind as judgments based upon these gifts. The direct data are only light, its intensity, color, and direction. These are incapable of further analysis, and are therefore simple sensations. Outline form may possibly be added, though this may be analyzed into a combination of directions. But solid form, size, and distance, though they may seem to be immediately perceived, are not direct perceptions, but only very simple judgments based on the data given above. We only

state these facts now that they may be borne in mind. We hope to substantiate them hereafter.

It is well also, in order to avoid confusion, to distinguish between light objective and light subjective— light as a vibration of the ether and light as a sensation. The one belongs to physics, the other to physiology. Our main concern is with light as a sensation; but since ethereal vibration is the cause of the sensation, we will have much to say of this also.

Primary Divisions of the Subject.

The whole subject of vision may be divided into two parts, viz., monocular and binocular vision. The former is simple vision without qualification. It includes the general phenomena characteristic of all vision. In addition to these there are certain other phenomena which are distinctive of the use of two eyes as one instrument. These constitute binocular vision. The distinction between these two kinds of phenomena will be fully explained in Part II.