so that the line of vision is directed to the right or left. The superior and inferior recti rotate the eye round the horizontal axis, and thus the line of vision is raised or lowered. The oblique muscles turn the eye round an axis passing through the centre of the eye to the back of the head, so that the superior oblique muscle lowers, while the inferior oblique raises, the visual line. It was also shown by Helmholtz that the oblique muscles sometimes cause a slight rotation of the eyeball round the visual axis itself. These movements are under the control of the will up to a certain point, but there are slighter movements that are altogether involuntary. Helmholtz studied these slighter movements by a method first suggested by F. C. Donders. By this method the apparent position of afterimages produced by exhausting the retina, say with a red or green object, was compared with that of a line or fixed point gazed at with a new position of the eyeball. The ocular spectra soon vanish, but a quick observer can determine the coincidence of lines with the spectra. After producing an after-image with the head in the erect position, the head may be placed into any inclined position, and if the attention is then fixed on a diagram having vertical lines ruled upon it, it can easily be seen whether the after-image coincides with these lines. As the after-image must remain in the same position on the retina, it will be evident that if it coincides with the vertical lines there must have been a slight rotation of the eyeball. Such a coincidence always takes place, and thus it is proved that there is an involuntary rotation. This minute rotation enables us to judge more accurately of the position of external objects.
3. The horopter is the locus of those points of space which are projected on retinal points. While geometrically it may be conceived as simple, as a matter of fact it is generally a line of double curvature produced by the intersection of two hyperboloids, or, in other words, it is a twisted cubic curve formed by the intersection of two hyperboloids which have a common generator. The curves pass through the nodal point of both eyes. An infinite number of lines may be drawn from any point of the horopter, so that the point may be seen as a single point, and these lines lie on a cone of the second order, whose vertex is the point. When we gaze at the horizon, the horopter is really a horizontal plane passing through our feet. The horopter in this instance is the ground on which we stand. Experiments show “that the forms and the distances of these objects which are situated in, or very nearly in, the horopter, are perceived with a greater degree of accuracy than the same forms and distances would be when not situated in the horopter” M.'Kendrick, Life of Helmholtz, 1899, p. 172 et seq.).
An object which is not found in the horopter, or, in other words, does not form an image on corresponding points of the retinae, is seen double. When the eyeballs are so acted upon by their muscles as to secure images on non-corresponding points, and consequently double vision, the condition is termed strabismus, or squinting, of which there are several varieties treated of in works on ophthalmic surgery. It is important to observe that in the fusion of double images we must assume, not only the correctness of the theory of corresponding points cf the retina, but also that there are corresponding points in the brain, at the central ends of the optic fibres. Such fusion of images may occur without consciousness—at all events, it is possible to imagine that the cerebral effect (except as regards consciousness) would be the same when a single object was placed before the two eyes, in the proper position, whether the individual were conscious or not. On the other hand, as we are habitually conscious of a single image, there is a psychical tendency to fuse double images when they are not too dissimilar.
4. Binocular Perception of Colour.—This may be studied as follows. Take two No. 3 eye-pieces of a Hartnack’s microscope, or two eye-pieces of the same optical value from any microscope, place one in front of each eye, direct them to a clear window in daylight, keep them parallel, and two luminous fields will be seen, one corresponding to each eye. Then converge the two eye-pieces, until the two luminous circles cross, and the central part, like a bi-convex lens, will appear clear and bright, while the outer segments will be much less intense, and may appear even of a dim grey colour. Here, evidently, the sensation is due to a fusion of impressions in the brain. With a similar arrangement, blue light may be admitted by the one eye-piece and red by the other, and on the convergence of the two, a resultant colour, purple, will be observed. This may be termed the binocular vision of colours. It is remarkable that by a mental effort this sensation of a compound colour may be decomposed into its constituents, so that one eye will again see blue and the other red.
6. The Psychical Relations of Visual Perceptions
1. General Characters of Visual Perceptions.—All visual perceptions, if they last for a sufficient length of time, appear to be external to ourselves, erect, localized in a position in space and more or less continuous.
(a) Visual Sensations are referred to the Exterior.—This appears to be due, to a large extent, to habit. Those who have been born blind, on obtaining eyesight by an operation, have imagined objects to be in close proximity to the eye, and have not had the distinct sense of exteriority which most individuals possess. Slowly, and by a process of education, in which the sense of touch played an important part, they gained the knowledge of the external relations of objects. Again, phosgenes, when first produced, appear to be in the eye, but when conscious of them, by an effort of imagination, we may transport them into space, although they never appear very far off.
(b) Visual Sensations are referred to Erect Objects.—Although the images of objects are inverted on the retina we see them erect. The explanation of the effect is that we are conscious not of the image on the retina, but of the luminous object from which the rays proceed, and we refer the sensation in the direction of these rays. Again, in running the eye over the object, say a tall pole, from base to apex, we are not conscious of the different images on the retina, but of the muscular movements necessary to bring the parts successively on the yellow spot.
(c) Visual Sensations are referred to a Position in Space.—The
localization of a luminous point in space can only be
determined by observing its relations to other luminous points
with a given position of the head and of the eye. For example,
in a perfectly dark room, if we look at a single luminous point,
we cannot fix its exact position in space, but we may get some
information of a vague character by moving the head or the
eye. If, however, a second luminous point appears in the darkness,
we can tell whether it is nearer or farther distant, above
or below the first. So with regard to other luminous points
we observe their reciprocal relations, and thus we localize a
number of visual impressions. There are three principal
directions in space: the transverse (breadth), the vertical
(height) and the sagittal (depth). Luminous points may be
localized either in the transverse or vertical directions.
Here we have to do simply with localization on a surface. A number
of points may be observed simultaneously (as when the eye is
fixed) or successively (as when the eye moves). If the movement
of the eye be made rapidly, the series of impressions from
Fig. 25.—Diagram illustrating
the Localization of Visual
Perceptions.
different points may be fused
together, and we are conscious of
a line, the direction of which is
indicated chiefly by the muscular
sensations felt in following it.
The case is different as regards
points in the sagittal direction.
We see only a single point of
this line at a time; it may be
a transverse series of retinal
elements, A B, and each of these
formed by a number of smaller
elements, 1, 2, 3, 4, situated in
the axis of each principal element;
it may be, on the other hand, the transverse line a b situated
in space and formed by a series of points in juxtaposition.
