retina. (Fig. 2.) If the luminous point be nearer than twenty feet, the refracting or bending effect of the eye must be increased (accommodated) so that

FIG. 2.

- NORMAL EYE AT REST, FOCUSED FOR DISTANCE.

the conjugate focus shall still be at the retina. (Fig. 3.) This is accomplished by increasing the thickness of the crystalline lens, shown by the dotted line, Fig.

FIG. 3. ACCOMMODATION FOR NEAR VISION BY INCREASED THICKENING OF THE LENS.

3. As before stated, when the eye is able to reproduce a luminous point in space as an irritant point on the retina, the optical requirements for perfect vision are secured.

Now "outward projection " means that the retina is touched at this mathematical point, and, like all other senses, it refers the sensation back to the source; in this case along the central line of the pencil of rays. The size of the image on the retina of the largest object that can be seen at all clearly at one time with

THE INVERTED IMAGE

49

out moving the eye or the object is probably not greater than three millimeters.

Conversely, the field of vision of clear vision with an immovable eye is extremely limited. At the length of the arm a circle the size of the thumbnail represents all that can be seen clearly, and it is only by rapid excursions that the eye sees in detail those portions that were only outlined before. The field of vision has been compared to a painting which is hazy and indistinct except a circle one-half inch in diameter, in which the most minute details are worked out. This small area may be any portion of the picture which is desired, by turning the eye toward that spot, but no two places at once. It is hard to believe this, for the eye, by rapid excursions, so quickly covers a large field that the separate sensations are fused into one.

Now, the analogy and bearing of this is important when it is understood that we do not see even this one-half inch object as a whole. Each mathematical point of which the object is composed sends out its bundle of rays, which are again converged to a point upon the retina, and from this irritation conveyed to

According to Suter: "Refraction and Motility of the Eye,” page 142. "The fovea centralis, upon which falls the image of every object attracting mental attention, does not exceed 0.4 mm. in diameter." Taking the distance of the nodal point in front of the retina to be 15 mm. (Dennett), by a very simple problem of similar triangles, it is demonstrable that, at a distance of one-half meter, an object to be discerned with normal acuity cannot exceed 13.5+ mm. in diameter.

the brain sensation (sight) results, which refers the irritant right back along the ray-line of each pencil to its source. So point after point irritates the retina, and is referred to its appropriate place in space until the luminous object is reproduced in the external world by the outward projection of an infinite number of luminous points.

To make this clear, a very simple object should be used; let it be a vertical line. (Fig. 4.) Now a lumi

B

FIG. 4.- THE OUTWARD PROJECTION OF THE RETINAL IMAGE.

nous point at the top of this line produces an irritant point at the lower side of the retina, which is referred back to its source above and seen there in space.

A luminous point at the bottom of this line produces an irritant point on the upper side of the retina, and is referred back to its position in space, which is below. A point from the center is referred back to its place in the centre for the same reason. And so with a point midway from the center to the top, and a point midway from the center to the bottom; and the process goes on simultaneously for each point of which the line is composed, and a sort of mental composite results, which is the exact counterpart of the object, occupying the identical position in space;

THE INVERTED IMAGE

5 I

somewhat as a spiritual body is conceived to be immanent in the natural body.

The solid lines represent the pencils of rays, the broken lines the axial rays of each pencil, showing the direction of the outward reference of the three irritant points illustrated.

To recapitulate: By the law of conjugate foci, a luminous point in space corresponds to an irritant point on the retina. By the law of outward projection it is referred to its proper place in the object, and, as the object is not seen as a whole, an infinite number of such luminous points of which the object is composed are referred to their respective positions, and furnish a synthetic conception, which must be erect because each of its constituent points is in its place.

Since Newton, scientists have recognized that for one body to act upon another at a distance, some medium must intervene. So with sight, the provisional ether is that intermediate something which reaches from the luminous point and "touches " the retina. The unlimited extent of the ether gives to sight unlimited range, and, altho light travels 186,000 miles per second, it takes three and a half years for light to reach us from the nearest

star.

Professor Dolbear says: "The light which reaches us to-day from some of the more distant stars left them before America was discovered; before Jesus was born; before the pyramids were built,

and, for all we should be able to see, they may have ceased to exist long ago, tho their light still shines." I

While this work was in press there appeared in Current Literature, September, 1906, an article entitled "A New Theory of Vision." This was a quotation from Cosmos, Paris, of the work of Mr. George Poullaine, who claimed to have discovered "a loop or twist in the optic nerve." The twist is in the protuberance of the outer and posterior parts of the optical layer of the brain. "The peculiar conformation explains," says Cosmos," the reinversion of the retinal image.

"The optic nerves, after emerging from the eyeballs, converge to the optic chiasma. Here they partly cross, or seem to exchange part of their fibers. The two nerve bundles thus modified separate and pass around the peduncles. In this part of their course they are known as the optical bands or Gasset's hemopic nerves.

"These bands enter the brain. Their fibers can be traced in the pulvinar, where they describe concentric curves. They can be traced also in other portions of the optical layer, where they are known as Gratiolet's optic rays.

"In order to more correctly ascertain the paths of the fibers, Poullaine studied and measured sections of the loop made by a horizontal plane and by two vertical planes, anterior, posterior, and transverse. 'Matter, Ether, and Motion.