the soul, because his dissecting knife failed to reveal anything of this nature. He would believe nothing that he could not see. Then the minister asks him if he can see a pain, and if not why he believes in pain. Of course he replies that he feels the pain, whereupon the clergyman replies that he knows of spiritual matters by other senses, which are (perhaps) in the doctor undeveloped.

The mental attitude which is undaunted by the restraints of the evidence of the senses is undoubtedly dangerous to the attainment of truth. To claim that one possesses transcendental royal roads to knowledge is intolerable to the scientific mind, yet a recognition of the limitations of our knowledge of the simplest fact must have a salutary effect upon our egotistic tendencies.

This relativity is especially apparent when we consider the special senses. For example, the membrana tympana is made to vibrate in unison with waves of air emanating from some sonorous body, but those waves must reach a velocity of sixteen per second before they are audible to the human ear. We have abundant evidence that the solar spectrum extends far beyond the visible spectrum. The wonderful development of the sense of smell in the dog simply baffles our comprehension. "Thus we learn," as Spencer says in his "First Principles," "the greatness and the littleness of the human intellect its power in dealing with all that comes within the range of experience; its

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impotence in dealing with all that transcends experience."

THE TACTILE SENSE

As the more delicate senses have evolved from simple sensitiveness to contact or the touch sense, it may be interesting to note that a simple refinement of this contact sense is evident thruout. Taste requires that the article brought in contact with the sense organ shall be soluble. In smell, which is closely allied, the contact of finely divided particles of matter is effected by their floating in the air. The matter may be as finely divided as the gaseous state. Hearing requires only the contact of air itself in waves, but still contact is here. The hammer-like form of the organ of Corti is very suggestive of this essential element contact.

The end organs of the optic nerve are the rods and cones of the retina. The contact here is the extreme refinement of touch. The ether wave initiated by the luminous body is condensed by the lenticular system of the eye into an irritant point, the focus, which touches the retina. This principle of contact and the fact that the special senses are merely a refinement of the same are of immense advantage in explaining some of our sight perceptions.

The other general principle which should be recognized is the outward reference of sensation. The common experience of striking the ulnar nerve at the elbow and feeling the sensation at the tip of

the little finger illustrates a general principle which applies to all the senses.

This outward reference may be made to extend beyond the finger. Tap the floor with a somewhat flexible cane with the eyes closed and we feel the contact at the end of the cane.

Perhaps we have all had the unpleasant experience of "seeing stars" from a blow on the eye. This is because an irritation of the optic nerve gives a sensation of light which is referred outward into space. This is also proof of the specialization of the optic nerve. Moreover, the testimony of those from whom it has been necessary to remove the eye without an anesthetic is that, when the optic nerve is divided, not pain but a flash of light is the resulting sensation.

This outward reference of the sense of hearing is one means of estimating direction. Notice the involuntary turning of the head so as to put the auricle at right angles to the sound wave. This is in obedience of the general law that all irritations reaching these special senses are referred back to their source.

SIGHT

The refinement of touch and outward reference reaches its highest development in sight.

Primitive eyes and ears bear a strong resemblance to each other. Later the eye has for its prototype the pigment spot. This suggests that sight developed first as an actinic susceptibility, the pigment spot absorbed the heat of the luminous body, and thus be

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came conscious of that which had heretofore been an unknown world.

It is necessary to touch upon the embryology of the eye only for its psychological bearing, that is, the development of sight. Notice then that the sensitive area becomes depressed, then cupped. This shape protects better and also increases the recognition of direction. For greater protection the cavity becomes closed and the cornea and lens develop. For what purpose? Evidently to collect the scattered rays of light and converge them, and focus them in a minute point on the retina. Acute sight demands perfect focusing.

Now let us apply the general law of sense perception and the outward reference of sensation to one of the psychological problems of sight, one which has been very generally misinterpreted.

The Inverted Image. — It is well known that the image on the retina is inverted. How does it happen that we see objects erect? The writer devoted considerable attention to this subject some years ago, ransacking all attainable literature.

Most of the physiologies agree that it is wholly the result of experience, that the child learns by touch to reinvert the retinal picture. Foster's explanation is as follows:

"As a matter of fact the field of vision, in one important particular, does not correspond to the field of external objects. The image is inverted. The rays of light proceeding from an object which by

touch we know to be on what we call our right-hand fall on the left-hand side of the retina. If, therefore, the field of vision corresponded to the retinal image the object would be seen on the left hand. We, however, see it on the right hand, because we invariably associate right hand tactile localization with left hand visual sensation. That is to say, the field of vision, when interpreted by touch, is a reinversion of the retinal image."1

Martin, in his work on "The Human Body," says: "A new-born child, even supposing it could use its muscles perfectly, could not seize a reachable object which it saw. It would not yet have learned that attaining a point exciting that part of the retina above the fovea (center) meant reaching a position in space below the visual axis; but very soon it learns that things near its brow, that is, up, excite certain visual sensations, and objects below its eyes, others; and learns to interpret retinal stimuli, so as to localize accurately the directions, with reference of its eyes to outer objects, and never henceforth gets puzzled by retinal inversion."

These two statements are fairly representative, and altho clear and lucid, are not only inadequate, but

erroneous.

First. This reversal by one sense, the tactile, of the testimony of the outer world, as given by another sense, the visual, is not analogous to the other special senses; and during the learning lapses would

' Physiology.