Albrecht von Graefe

“ Even an eye whose sight is quite normal, makes an analogous observation if it directs its gaze to a very fine point of light, as, for example, to a star. Both the star and the atmosphere are equally innocent of the small beams that radiate from it; they are the rays of our own lens which we transplant to heaven. So little aware are we of what takes place in the depth of our sensory organs, or in the immeasurable distances of the universe.
The spaces between the lens and the cornea, as also between the lens and the retina, are filled with a liquid medium, called humors ”

“ The blind spot is by no means so excessively small. At the distance of four paces, it would cover a man's head in the centre of your field of view; and almost a hundred moons in the sky would find room within its bounds. ”

“ The senses are indeed the gates to the mind, through which aliment passes for its sustenance; but equally, they are the portals through which science must endeavor to penetrate into the mental world. This has often been attempted, though in another manner, as by laying down postulates on the nature of the soul. ”

“ Some years ago, I was happy in being able to demonstrate that a comprehensive range of diseased conditions and also blindness, the causes of which had been successively sought for in different parts of the eye, simply arose from too great a tension being exercised by the vitreous humor—a discovery which imparted so much the more pleasure, as a suitable remedy was likewise at hand. ”

“ And since men have applied themselves to trace with care the psychical manifestations of perception into the world of ideas, and to do the utmost in their power to discover the laws which there govern, there has arisen another science, forcing itself more and more on our notice as it daily proves itself to be possessed of an inherent vitality. ”

“ We cannot entirely overlook the question whether sensations of light can be produced with the assistance of any other mechanism in the body but that of the visual nerve. As only the part where sensation originates is endowed with specific sensory action, so the irritation of that part can alone produce impressions of vision; but this irritation may be imparted to it by other parts of the brain, or by other nerves. It has been already stated that irritations of the brain produced by narcotics are transmitted only by proximity to the terminal extremity of the optic nerve. ”

“ Here the eye, as an optical apparatus, remains passive. Just as a man, who sees, is aware of the phenomena even in the deepest darkness, so is also a blind man, as long as the retina is endowed with its specific sensitiveness or sensory power, by which it responds to every irritation with a luminous sensation. Even after blindness, this dancing of sparks of fire and light may be kept up by continual irritation in the eye to such a tormenting extent, that, in order to prevent it, we usually cut the optic nerve just behind the eye, when the sparkling and scintillating cease. ”

“ You have yourselves, consciously or unconsciously, often made such experiments, when you watched the circles and sparks of fire and light, which become visible on rubbing or pressing your eyes through their lids, or striking them with a hard substance. ”

“ According to physics, light is the same species of motion in the ether as warmth; only, in order to affect the retina, the undulations must take place within certain limits of rapidity. Relatively they possess the greatest velocity in the violet-colored part of the spectrum, and the least in the red. In the same proportion as the velocity of the undulations diminishes, does light become invisible, and only dark rays of warmth are emitted, while on its being more highly heated, from the increased rapidity of the undulations, it reaches the glowing-point, that is, it emits rays of red light. ”

“ Indirect or eccentric vision, as it is termed, makes us aware simply of the presence of objects, by giving us some notion of their shapes; but we are unable to distinguish even the biggest letters, if the image of them should fall only one hair's breadth off that one spot in the retina. In reading, the eye must constantly move onward to the end of the line, the single letters thus gradually imprinting themselves on the direct point of vision. On the other hand, indirect vision offers hints for fixing the object in our eye ”

“ A general irritation of the retina unaccompanied by a perception of objects will give us a light field of view, and, on the other hand, a perfect repose of those parts will give us a dark field of view. The former represents the sense of the repose of a mechanism endowed with the power of action; the latter corresponds to the absence of all mechanism whatever. The feeling of darkness, therefore, results merely from the expansion of your field of view as opposed to your retina, if I may so say; while behind your back you have the feeling neither of light nor of darkness ”

“ A child will assuredly not appreciate the difference between the pencil and the trunk of the tree in the same degree as an individual who by experience has learned to know the value of his impressions. ”

“ Now, while the light coming from external objects irritates the retina variously according to its color and power, the impressions made by luminous objects are also very various, and herein lies the first link of connection with the outer world.
It is only the visual organs of the lower animals which lose themselves in such a general and vague relation to the surrounding ocean of light and color. The organ which now occupies our attention has a far higher design to serve, viz., to awaken a perception of separate objects, and of their peculiar forms and colors. ”

“ Essential and most important progress has been made in the knowledge of the human body, since men have ceased to indulge in subtle speculation into the nature of the principle of life, and turned with an undivided spirit of inquiry into the laws of organic appearances. ”

“ If it has a wide slit, it affords a view over a greater part of the eyeball, and we think it bigger, simply because we see more of it. In the same manner, our judgment is deluded by the different degrees of prominence of the eye. A staring or protruding eye impresses us as being larger, although it is only pressed forward; while in advanced age, or in consuming sickness, the sunken eye is thought smaller. ”

“ Should the lenticular elasticity no longer admit of a sufficient scope for refraction, we must then either adjust the distance of the objects, as we see a far-sighted individual do, by holding the book proportionably farther off, or we must afford the eye assistance by accommodating it with movable auxiliary lenses, spectacles, which replace the lost power of adjusting forms to the natural eye. This power of accommodating the focus disappears beyond recall if the lens has sustained an injury, or if we remove it entirely, from its having grown turbid. ”

“ The distance from the spot of direct vision, at which the objects may be perceived by eccentric vision, has its limits. When looking straight before you, you can just perceive a hand which stretches down the whole length of the face, in the direction of the temples. This is the extremest point from which it is possible for light to fall on the retina; but, if you attempt to go beyond it, the hand disappears, its image not being projected on the retina. ”

“ The eye of a man of sixty, that sees distinctly at a distance, is unable to distinguish an object five inches off; and, if, against all rule, it see sharply at five inches off, this benefit is counteracted by the inconvenience of seeing distant objects with great indistinctness, The gradual hardening of the lens is accompanied by a decrease of power of adjusting its focus, and is even subject to such small individual fluctuations that, by an exact calculation of its play, [3] we may sometimes arrive at the most indiscreet conclusions respecting the age. ”

“ BE the idea what it may, that we form to ourselves of the mysterious tie that links our perception to the life of the soul, so much is undoubted, that the material supplied by the impressions of the senses constitutes the basis on which the soul unfolds ”

“ Great as are the influences of this subjective sight for the refreshment of our brain during sleep, and powerfully as they affect the temperament of the blind, they cannot connect us with the outer world. The yellow light which floods our field of vision, on rubbing the retina, is of no use to light up external objects. ”

“ Unimportant irritations, which on the surface of the body are not noticed, seriously affect the cornea.
Let us now notice the second coat, the choroid. This we compared to the black coating of paint in the camera-obscura. Thinking of the dazzling and delusive visions which are a consequence of the gradual consuming away of the pigment in the choroid, or which accompany the entire want of it, as with the albinos, we cannot doubt that one essential design of this tissue is to intercept the scattered rays. ”

“ This invention exercises an influence not only on the peculiar branch of the oculist, but likewise on the broad field of medical investigation, seeing it affords an insight into the optic nerve, a direct process of the brain, and other structures, which, along with their analogies, were hidden from observation in the body.
The dimensions of the eyeball among strong-sighted individuals is more equal than you perhaps imagine. ”

“ In order to effect this, a great degree of elasticity, chiefly of the outer capsule, was obviously necessary, and we find this requisite complied with by an admirably delicate structure of concentric layers, according to which its density reaches the minimum at the periphery of the lens, while its aggregate power of refraction is increased, as if it were composed entirely of the strong refracting substance of which the centre consists.
As the power of accommodating its focus rests on this quality of the lens, it is necessarily accompanied by a loss of elasticity with increasing age. ”

“ The light with very divergent rays, and proceeding from near objects, is collected to a picture behind the lens, while that of the distant objects falls with almost parallel lines. To return to the camera-obscura, you must draw out the tube with the lens, in other words, remove the latter farther from the intercepting plate if nearer objects are to be impressed, and on the other, push it in, if more distant ones are wanted. ”

“ All those operations, therefore, which proceed from direct irritation of the nervous part of the visual organ, without the medium of the eye and of light, under the term subjective vision, are opposed to those phenomena produced by the media of eye and light, and known as objective vision. ”

“ You ask how men have arrived at the knowledge of these things, seeing the mechanism in question is almost entirely removed from any direct investigation. First, then, the umbellar expansion of the optic nerve, the retina, enables us to make experiments; this retina being in such close contact with the eye, the optical part of the visual organ, that it is accessible to every sort of mechanical irritation. ”

“ The surfaces of the lens give back extremely delicate reflections, which with the proper aids may be measured in the living eye, and from the size of which the curve may be calculated as in convex and concave mirrors. ”

“ By a simple experiment it is possible to make every person acquainted with these guests of his field of view, the so-called muscæ volitantes; only, one must be prepared for those formerly overlooked, but once honored with attention, never again stirring from their post. ”

“ By means of a process of collecting and comparing, compound ideas are evolved out of simple ones, and the normal, logically organized mental life attains an ever-higher development, while, by the inexhaustible activity of the senses, it receives a never-failing supply of fresh material for the perfecting of its psychical structure. ”