Reticulation

“ Our ideas are movements of the nerves of sense, as of the optic nerve in recollecting visible ideas, suppose of a triangular piece of ivory. The fine moving fibres of the retina act in a manner to which I give the name of white; and this action is confined to a defined part of it; to which figure I give the name of triangle. And it is a preceding pleasurable sensation existing in my mind, which occasions me to produce this particular motion of the retina, when no triangle is present. ”

“ Now if the change or motion of the retina was a mechanical impression, or a chemical tinge of coloured light, the perception would every minute become stronger and stronger,—whereas in this experiment it becomes every instant weaker and weaker. The same circumstance obtains in the continued application of sound, or of sapid bodies, or of odorous ones, or of tangible ones, to their adapted organs of sense. ”

“ It is evident, therefore, that the pictures upon the retina are, by the laws of nature, a mean of vision; but in what way they accomplish their end, we are totally ignorant. Philosophers conceive, that the impression made on the retina by the rays of light, is communicated to the optic nerve, and by the optic nerve conveyed to some part of the brain, by them called the sensorium; and that the impression thus conveyed to the sensorium is immediately perceived by the mind, which is supposed to reside there. ”

“ Perhaps a friend calls our attention to the fact that a balloon is sailing through the air, or some fine morning, hearing in the fields the blithe song of the sky-lark, we look up and vainly try to bring the small image upon the place of distinct vision. Now, if an image which falls upon any other part of the retina is perceived, even indistinctly, an instant suffices to direct the eyes into the exact position requisite for clear vision—an example of the marvellous precision with which impressions are put in relation to each other by the unconscious action of the brain. ”

“ THE retina is the point where the physical process of vision passes into the physiological process. Until it impinges upon the retina, the light which penetrates the eye has only undergone physical changes, consisting chiefly in refraction, the last perceptible result of which is the production of the image upon the retina. ”

“ The image is extremely minute, but owing chiefly to the disturbances of our atmosphere, the genuine image is surrounded with a region in which the nerves of the retina are more or less affected by the light, which ought, if all the sources of disturbance could be excluded, to be entirely concentrated within the image itself. ”

“ But, if in place of looking at the solid object itself we look with the right and left eyes respectively at pictures of the object corresponding to those which would be formed by it on the retina of the two eyes if it were placed at a moderate distance in front of them, and these visual pictures brought into coincidence, the same conception of a solid form is generated in the mind just as if the object itself were there. ”

“ If, instead of the photograph we have before our eyes a statuette, each eye will see a quite different view: the right eye will see a portion which is invisible to the left eye, and vice versâ, and, in fact, we shall see more than half round the object. Here, then, we have certain differences of the retinal pictures when solid objects are viewed, and these differences by innumerable repetitions have, unconsciously to ourselves, become associated with notions of solidity, of something having length, breadth, and depth, or thickness. ”

“ Rectificā′tion, the act of rectifying or setting right: the process of refining any substance by repeated distillation: rectification of a globe, its adjustment preparatory to the solution of a proposed problem; Rec′tifier, one who corrects: one who refines a substance by repeated distillation.—Rectify the course of a vessel, to determine its true course from indications of the ship's compass, and allowing for magnetic variations, &c.; Rectify the globe, to bring the sun's place in the ecliptic on a globe to the brass meridian. ”

“ And what is true of sight is equally true of all the other senses: if any stimulus is increased, the smallest increase of sensation first occurs when the stimulus rises one per cent, above its original intensity. Such being the law on the side of sensation, suppose that we place upon the optic nerve of an animal the wires proceeding from a delicate galvanometer, we find that every time we stimulate the eye with light, the needle of the galvanometer moves, showing electrical changes going on in the nerve, caused by the molecular agitations. ”

“ The images of any object impressed by light upon the eye are fugacious, and can be of only a limited intensity. The images fixed upon the photographic plate are permanent, and can be made of an intensity that becomes cumulative with the duration of the exposure. The photographic retina may be expected, when the art has been perfected in the highest degree, to give us images corresponding with an extremely expanded range in the duration of the exposure. ”

“ Now in the present state of our knowledge we cannot assert that in both the first and the last occurrence of the retinal image in question the same pure sensation is provoked, but that the mind interprets it differently the last time in consequence of experience; for the only given things we know are on the one hand the retinal image which is both times the same, and on the other the mental percept which is both times different; of a third thing, such as a pure sensation, interpolated between image and percept, we know nothing. ”

“ For the purposes of this paper therefore the reader will kindly assume refraction to stand for that intimate effect of one mass of color or value upon its adjoining mass which results in the "lost-edge," and a general diffusion of tone, thus giving to pictures their atmospheric quality.
Now refraction is only in a very limited sense an objective fact. It is mainly a visual fact whose operation is due to the imperfect construction of the lens of the human eye. The scientific fact is that the edges of things are sharp and hard as a rule. ”

“ It can be taken as a fact, which the theory of the action of the eye implies, that for each external impression, that is, for each image produced upon the retina, the ends of the visual nerves, concerned in the conveyance of the impression to the mind, must be under a peculiar stress or in a vibratory state. ”

“ Whence it is reasonable to conclude, that the light of the day must be much too weak in its dilute state to make any mechanical impression on so tenacious a substance as the retina of the eye.—Add to this, that as the retina is nearly transparent, it could therefore make less resistance to the mechanical impulse of light ”

“ The Stereoscope achieves its effect by blending into one image pictures or drawings of an object taken from slightly different points of view so that the impression of relief is obtained in our sense of vision. ”

“ From these experiments and observations, it will be seen that the conversion of form, excepting in the normal case, depends upon various causes, which are influential only under particular conditions, such as the depth of the hollow or the height of the relief, the distance of the object, the sharpness of vision, the use of one or both eyes, the inversion of the shadow, the nature of the object, and the means used by the mind itself to produce the illusion. ”

“ I have referred to the fact that sometimes on the cessation of light, an after-oscillation is observed, which may correspond to the after-oscillations of the retina, and give a probable explanation of the phenomena of recurrent vision. When we have looked at a bright object for some time with one eye, we find, on closing both eyes, that the image alternately appears and disappears. ”

“ Although the blurred and indefinite retinal picture may be compensated, and perhaps more than compensated, by the readiness with which the eyes move, it is, of course, possible that greater precision and delicacy of visual impression over the whole surface of the retina might be consistent with a still greater increase of our powers of perception. There are instances in which the absence of finish, as it may be termed, in all but one little spot in the picture, proves a real inconvenience and a sensible deprivation. ”

“ In making a general retrospect, finally, of the effects produced by stimulus of light, we find that there is not a single phenomenon in the responses, normal or abnormal, exhibited by the retina which has not its counterpart in the sensitive cell constructed of inorganic material. ”

“ There is a whole batch of illusions which come from optical sensations interpreted by us in accordance with our usual rule, although they are now produced by an unusual object. The stereoscope is an example. The eyes see a picture apiece, and the two pictures are a little disparate, the one seen by the right eye being a view of the object taken from a point slightly to the right of that from which the left eye's picture is taken. Pictures thrown on the two eyes by solid objects present this sort of disparity, so that we react on the sensation in our usual way, and perceive a solid. ”

“ For consider the course of the reaction experiment in the light of our typical formula of action! The observer sits down with the intention of moving when he has perceived the stimulus; and he has an idea of the [239] result of his movement, namely, the performance of a reaction experiment. The stimulus is presented; he perceives the object of movement; and slips his finger from the key. He thus perceives the movement itself, and also, by the movement, realises in perception his idea of result. ”

“ As an object becomes perceptible to us, by means of the image thereof which is formed on the retina, it will, therefore, be seen in that direction, in which the rays enter the eye to form the image, and will always be found in the line, in which the axis of a pencil of rays flowing from it enters the eye. ”

“ We hear if the ear is stimulated, etc. In Chapter VII we learned that after the brain has had an excitation giving rise to sensation, it is capable of reviving this excitation later. This renewal or revival of a brain excitation gives us an experience resembling the original sensation, only usually fainter and less stable. This revived experience is called image or idea. The general process of retention and revival of experience is, as we have seen, known as memory. An idea, then, is a bit of revived experience. A perception is a bit of immediate or primary experience. ”

“ The simple image may therefore be defined as an elementary mental process, akin to sensation and perhaps indistinguishable from it, which persists when the sensory stimulus is withdrawn or appears when the sensory stimulus is absent. We may say further that, while every normal person has very much the same equipment of sensations, there are great individual differences in the matter of secondary sensations or images; in some cases they are interwoven into the whole tissue of experience, in others they are infrequent or even lacking ”

“ In a vast variety of cases, images are accompanied by “accessory images,” as when we see bright objects in a looking-glass.26 Now, when an image is displaced by refraction, the displacement is not complete, clear and sharp, but incomplete, so that there is an accessory image along with the principal one.27 From these principles, the colors produced by refraction in the image of a bright object on a dark ground, are at once derivable. ”

“ Uses of Refraction. If it were not for refraction, or the deviation of light in its passage from medium to medium, the wonders and beauties of the magic lantern and the camera would be unknown to us; sun, moon, and stars could not be made to yield up their distant secrets to us in photographs; the comfort and help of spectacles would be lacking, spectacles which have helped unfold to many the rare beauties of nature, such as a clear view of clouds and sunset, of humming bee and flying bird. ”

“ Stimulus of light.—Even the responses of such a highly specialised organ as the retina are strictly paralleled by inorganic responses. We have seen how the stimulus of light evokes in the artificial retina responses which coincide in all their detail with those produced in the real retina. This was seen in ineffective stimuli becoming effective after repetition, in the relation between stimulus and response, and in the effects produced by temperature ”

“ I have been studying the refraction of the human eye for more than thirty years, and my observations fully confirm the foregoing conclusions as to the uselessness of all the methods heretofore employed for the prevention and treatment of errors of refraction. ”

“ And in general we find by other Experiments, that when the Rays which differ in Refrangibility are separated from one another, and any one Sort of them is considered apart, the Colour of the Light which they compose cannot be changed by any Refraction or Reflexion whatever, as it ought to be were Colours nothing else than Modifications of Light caused by Refractions, and Reflexions, and Shadows. ”

“ It now does not seem improbable that, when by the power of thought an image is evoked, a distinct reflex action, no matter how weak, is exerted upon certain ends of the visual nerves, and therefore upon the retina. ”

“ Reflexion.—When rays of light fall upon a plane surface, as the flat surface of the mirror, a greater or less number of them are reflected, and this according to a definite law, by which the angle of incidence, or that formed by the incident ray with the perpendicular, is equal to the angle of reflexion, or that formed by the reflected ray with the same. ”

“ If we judge from analogy in this case, it will lead us to think that there is a certain correspondence between points of the two retintv in such animals, but of a different kind from that which we have found in human eyes. The centre of one retina will correspond with the centre of the other, in such manner, that the objects whose pictures are formed upon these corresponding points, shall appear not to be in the same place, as in human eyes, but in opposite places. ”