Naturalistic Photography for Students of the Art.

by P. H. Emerson

Macula lutea.

In addition to this the macula lutea is less sensitive to weak light than other parts of the retina. The effect of all these imperfections is to blur and dull the perfect image. The serious defects due to the blind spot are not noticed, according to Helmholtz, because “we are continually moving the eye, and also that the imperfections almost always affect those parts of the field to which we are not at the moment directing our attention.” The italics are ours. Here, then, is another great difference between the eye and the optician’s lens.

Focussing.

The focus of the eye in a passive state is adjusted to the most distant objects. It focusses for nearer objects by contracting the ciliary muscle which pulls tight the zonule of Zinn and so curves the crystalline lens. It can focus thus up to within five inches of itself, but the changes of focus are almost imperceptible to the eye beyond twenty feet. Now a theoretically perfect eye might form perfect images of objects at infinite distances when there were no intervening objects. But as has already been shown, the eye is very imperfect, and its images are not therefore perfect, and it could not form theoretically perfect images, even if the atmosphere were pure ether and nothing else, for there are other facts in nature which prevent this; thus we cannot see a sharp image of the sun with the naked eye on account of its dazzling brightness.

Fovea centralis.

This central spot is a most important factor in the study of sight and art. For though the field of vision of the two eyes is more than 180° laterally, and 120° vertically, yet the field of distinct vision is but a fraction of this field, as we can all prove for ourselves. Now the field of distinct vision depends on the central spots for the reason that the central spot differs anatomically from the rest of the retina by the absence of certain layers which we need not specify here. The absence of these layers exposes the retinal bacillary layer to the direct action of light. Helmholtz says “all other parts of the retinal image beyond that which falls on the central spot are imperfectly seen,” so that the image which we receive by the eye is like a picture minutely and elaborately finished in the centre, but only roughly sketched in at the borders. But although at each instant we only see a very small part of the field of vision accurately, “we see this in combination with what surrounds it, and enough of this outer and larger part of the field, to notice any striking object, and particularly any change that takes place in it.” If the objects are small, they cannot be discerned with the rest of the retina, thus, to see a lark in the sky, Helmholtz says it must be focussed on the central spot. Finally he says, |Direct and indirect vision.| “To look at anything means to place the eye in such a position that the image of the object falls on the small region of perfectly clear vision. This we may call direct vision, applying the term indirect to that exercised with the lateral parts of the retina, indeed with all except the central spot.” Again, he says, “Whatever we want to see we look at and see it accurately; what we do not look at, we do not as a rule care for at the moment, and so do not notice how imperfectly we see it.” Now all this is most important in connection with art, as we shall show later, we must beg the student therefore to hold it fast.

It will be seen from all this that a perfect periscopic image is never seen by the eye of man, though in some of the lower animals the matter may be different.

B. Direction of Light.

Law of projection.

Le Conte says, “The retinal image impresses the retina in a definite way; this impression is then conveyed by the optic nerve to the brain, and determines changes there, definite in proportion to the distinctness of the retinal image, and then the brain or the mind refers or projects this impression outward into space as an external image, the sign and facsimile of an object which produces it.” Not only does this hold good of external images, but in certain diseases retinal impressions arising from within are projected outwards, thus ghosts are seen.