Compound eye

“ The phenomena of ocular spectra and complementary colours experienced by every one forms a curious chapter in the history of those illusions which take their origin in the eye itself. Every one has noticed that after looking fixedly at a bright light or a striking colour for a few moments, the eye preserves an impression of the object for a certain time. A very light window looked at intently for several seconds will leave the impression of its cross-bars on the retina for several minutes, the colour of the image changing at every movement of the eye. ”

“ This consequence of a common and yet subsidiary vision of these facets, springs partly from the immobility of the eyes, and partly from the circumstance that only those rays of light which fall in a right line upon a facet of the eye, which itself forms the segment of a circle, can reach the optic nerve of this facet, whereas all others are withheld by the pigment which partly separates the individual glass lenses from each other, and partly surrounds the margin of the chrystalline lens, beneath the cornea. ”

“ The seat of vision is generally supposed to be the retina, though some philosophers regard the choroid coat as the sensitive tablet upon which the impression is made. We may trace the phenomena of vision up to this point, but no further. We know that a distinct image is formed upon one or other of the delicate coats of the eye, but the manner in which the sensation is conveyed to the brain is an inscrutable mystery. ”

“ Not only this: the spot being always situated on the outer and indistinct portion of the image reflected on the retina, we do not take notice of it; for as every one has no doubt observed, it is only the small portion of the object we are looking at exactly opposite the centre of the eye that is perfectly distinct and clear, all the rest being confused in its details, although quite visible. ”

“ In September, when I first saw him, his left eye was very much diminished in size, and the cornea studded with white spots; the vision very imperfect, and constantly discharging water. The right eye was so opaque that neither iris nor pupil was to be seen, and he could only see the shadow of an object. ”

“ There we see the effect of the crystalline lens of the eye in bringing the rays to a focus within a much shorter distance than that which would be required had there been present only the curved cornea, and the aqueous and vitreous humours of the eye, which are but little different from pure water in their optical properties. ”

“ In the normal human eye, only the central parts of the retina are sensitive to colour, the peripheral parts are practically colour-blind. Anæmia of the retina, which may be produced by pressure on the eye-ball, will render the retina, first colour-blind and then insensitive to light. ”

“ Were it not for the retina there would be no color; and when the sensory nerves of the retina are partially paralyzed or deficient, as in the case of the color-blind, nature appears to the eye in her true monochromatic garb.
The human eye resembles closely the photographic camera, both in structure and in its manner of functioning. ”

“ The black eye certainly labours under one disadvantage, which is, that, from the iris and pupil being of the same colour, the contraction and dilatation of the latter is not seen, by which the eye is abridged of half its powers. ”

“ The eye will accommodate itself to light of different degrees of intensity within a limited range, but both eyes should be exposed to an equal degree of light. The sympathy between the eyes is so great, that if the pupil of one eye is dilated by being kept in the shade, as must, of course, be the case where the light is on one side, the eye which is exposed can not contract itself sufficiently for protection, and is almost inevitably injured. ”

“ The pupil of the eye is seen to dilate, or contract, in proportion to the quantity of light it receives, yet is incapable of distinguishing objects, because the organs of vision are still imperfect; the tunica cornea, or horny tunicle is wrinkled, and perhaps the retina is also too soft to receive the images of external objects, and admit the sense of seeing. ”

“ We are so accustomed to merely play with children, that we are easily blinded to the full seriousness of such requests.
We cannot break off these reflections on the image of the retina without making mention of a remarkable spot in the background of the eye—the yellow spot marking the entrance of the optic nerve. All perception whatever is arrested within the bounds of this spot—it is a blind point in our field of view. ”

“ By degrees the surface of the eye assumes a deeper red, it loses its brightness and its polish, while the swelling of the lids lessens, and they can be opened with less difficulty; their inner surface at the same time becomes softer, but thick and granular, and next the eyes themselves put on likewise a granular condition which obscures vision. ”

“ The capability of the eye in the delicate discrimination of shades of direction and distance, and still more in the coördination of manifold details under some aspect of unity, seems to be inseparably bound up with the fact of simultaneous retinal impression. ”

“ No matter whether the object regarded is strange or familiar, whether the light is good or imperfect, whether the surroundings are pleasant or disagreeable, even under conditions of nerve strain or bodily disease, the normal eye is expected to have normal refraction and normal sight all the time. ”

“ Aristotle, however, says that this fact is to be explained in another way: the iris of the eye shines like a phosphorescent substance; when the eye is struck, the sudden shock of the blow causes the eye as an object of vision to become separate from the eye as the organ of vision, and thus the eye for an instant sees itself! ”

“ The left eye is the reflexion of the right. And the light-feeling retina of the left eye is a reflexion of the light-feeling retina of the right, in all its functions.
The lense of the eye, like a magic lantern, casts images of objects on the retina. And you may picture to yourself the light-feeling retina of the eye, with its countless nerves, as a hand with innumerable fingers, adapted to feeling light. The ends of the visual nerves, like our fingers, are endowed with varying degrees of sensitiveness. ”

“ Lens-like structures, formed either as a thickening of the cuticle, or as a mass of cells, were subsequently formed; but their function was not, in the first instance, to throw an image of external objects on the perceptive part of the eye, but to concentrate the light on it. From such a simple form of visual organ it is easy to pass by a series of steps to an eye capable of true vision. ”

“ For example, if the eye, adjusted to a dim light, is suddenly turned toward a brilliantly lighted object, the retina will receive too much light and will be shocked before the muscles controlling the iris can react to shut out the superabundance of light. ”

“ It is this impression of light on the optic nerve which gives our ideas of light and colors.
The eye is also formed in such a way that it can alter its shape and become somewhat oblong, while at the same time its lens is projected forward or drawn back. The object of this contrivance is to obtain an equally perfect picture of distant and of near objects.
Our ideas of shape and size at first are not gained by the eye, but by the sense of touch. After considerable experience we learn to determine shape and size by the eye. ”

“ The internal structure is still more admirable. The globe of the eye is composed of tunics, humours, muscles, and vessels; the coats are the cornea, or exterior membrane, which is transparent anteriorly, and opake posteriorly; the charoid, which is extremely vascular; the uvea, with the iris, which being of various colours, gives the appearance of differently coloured eyes; and being perforated, with the power of contraction and dilatation, forms the pupil; and, lastly, the retina, being a fine expansion of the optic nerve, upon it the impressions of objects are made. ”

“ The human eye is so constituted, that it can only have distinct vision, when the rays which fall on it are parallel, or nearly so; because the retina, on which the image is painted, is placed in the focus of the crystalline humor, which performs the office of a lens in collecting rays, and forming the image in the bottom of the eye. ”

“ We may expect by now that our eye is fitted with two modes of seeing activity, polar to each other, and that the way in which they come into operation depends on whether the interplay of positive and negative density outside the eye leads to the appearance of the blue-violet or of the yellow-red side of the colour-scale. Such a polarity in the activity of the eye can indeed be established. ”

“ Nervous movements of the eyelids may also be noted, either by ordinary inspection, or by lightly touching the lid of one eye while the other regards an object either at the near point or the distance. The visual axes are never parallel, and the deviation from the normal may become so marked as to constitute the condition of squint. ”

“ This final appearance of homogeneity may perhaps be facilitated by the fact that in binocular vision two points situated on the prolongation of the optical axis of one of the eyes, so that the near one hides the far one, are by the other eye seen laterally apart. Each eye has in fact a foreshortened lateral view of the other's line of sight. ”

“ Having often observed in the dissections of men, as well as of brutes, that the optic nerve is not inserted exactly opposite to the pupil, that is, in the place where the picture of the objects upon which we look directly, is made: and that in man it is somewhat higher, and on the side towards the nose, he had the curiosity to examine the reason of this structure, by throwing the image of an object on this part of the eye. ”

“ If you will bear in mind the fact that the colors and values that are seen out of the corners of the eyes, are, on account of their very situation, able to affect only a very limited number of the sensitive nerves of the retina, you will understand that the force of their impact must be proportionately less than those which come to the eye from the full centre of vision ”

“ Next we examine the inner corner of the eye (nearest the nose) , where are the soft pink tear-ducts, which we touch in lightly with the pencil.
The outer corner of the eye forms a sharper angle, with the upper lid curving downward to meet the flesh of the lower lid and its corresponding upward curve.
Having sketched the shape of the eye as a whole, we might next return for a closers observation of the iris.
Is there anything in nature more lovely than the iris of the human eye? The liquid tint of blue, grey, or brown is like the luminous colouring of a strange flower. And it is colour. ”

“ Of all the senses none is more frequently the seat of such deceptive judgments than that of sight; and in the science of physiological optics a very considerable share of attention is claimed by optical illusions. For the purposes of convenience, we may draw a distinction between these illusions, which are the direct result of certain properties or imperfections of the eye as an optical instrument, and those which arise from obliquities of judgment in interpreting the sensations optically impressed upon the retina of the eye. ”

“ Nor is it at all difficult to understand how this general condition comes to be associated with the eye, and how the particular visual sensations come to be associated with something distant from the eye: and further, how this association of the condition with one thing, and of the sensations with another thing, (an association established by the touch and not by the sight,) dissolves the primary synthesis of seeing and colours. ”

“ When there is any malformation, either in quality of refraction, media, or mechanical parts, or adjustment of the eye, short sight, or other imperfections, and even blindness are the result. Even light, or rays which enter the eye, cannot be used by all eyes alike, but to some they create indistinctness and even pain. ”

“ We answer, by teaching us to associate vision in general, or the abstract condition regulating our visual impressions, with the presence of the small tangible body we call the eye, and vision in particular, or the individual sensations of vision, (i.e. colours,) with the presence of immeasurably larger bodies revealed to us by touch, and tangibly external to the tangible eye. Sight, as we have said, does not inform us that its sensations are situated in the eye: it does not inform us that we have an eye at all. Neither does touch inform us that our visual sensations are located in the eye. ”

“ This mechanism of the iris in diminishing or enlarging the pupil can be neatly observed by looking into a mirror placed on one side of a window. If the hand is put up in such a way as to screen the eye from the light, the pupil will be seen to enlarge ”

“ Unless the eye is exactly focused at the point from which it is being observed, one sees also a dark shadow at the edge of the pupil, and it is the behavior of this shadow when the mirror is moved in various directions which reveals the refractive condition of the eye. ”

“ Its finest fibers are ultimately connected with numerous elongated cells which are stimulated by light. The retina is dark purple in color, this color being caused by a layer of cells next to the choroid coat. This [Pg 361] accounts for the black appearance of the pupil of the eye, when we look through the pupil into the darkened space within the eyeball. The retina acts as the sensitized plate in the camera, for on it are received the impressions which are transformed and sent to the brain as sensations of sight. The eye, like the camera, has a lens. ”