the stimulus) the after-physiological process which takes place in the eye may be like that aroused by the original light stimulus, or like that aroused by its complement. It can readily be seen that the principles involved in the development of the negative after-image account certainly for the phenomenon known as successive contrast, and possibly for that known as simul- taneous contrast.
Phenomena Known as Successive and Simultaneous Con- trast.— Successive contrast is a phenomenon which plays an enor- mous role in the sphere of the artist, of the modiste and of the milliner. If two strips of colored cloth reflecting, for example, 92 PSYCHOLOGY red and green rays, which are, of course, complement a^y, are placed side by side, the stimulating value of each along the line of junction is greatly enhanced. We saw above that there was a tendency. for the eye to become complementary sighted when stimulated by a given light. The same phenomenon appears here. The strip of the retina excited by the green rays tends to de- velop the negative after-image process. When the strip is shifted by reason of the unsteadiness of the eye so that the red rays actually fall upon it, the two processes are summed. The same process takes place in the strip excited by the red rays.
Simultaneous contrast is a curious phenomenon. It can be most easily observed in a dim room with spectral light. Allow any monochromatic light to fall upon a narrow strip of plaster of Paris, and throw a shadow across the center of the beam with a pencil or other narrow object. Theoretically, where the shadow falls there is no monochromatic light stimulation. The subject, however, reacts to it exactly -as though we had thrown the com- plementary light upon the shaded area. It can be very easily observed by pasting a gray strip two cm. square upon a sheet of colored paper and covering the whole with thin tissue paper.
Effect of a Stimulation of the Peripheral Retina with Mono- chromatic Light. — So far in our discussion we have dealt with the responses which appear when the central area of the retina is stimulated. The regions outside of the center of the eye can be stimulated by having the subject gaze at a fixation mark with the center of the eye. The experimenter then gradually introduces a monochromatic stimulation from behind. He introduces the light on the arc of a circle, keeping it equidistant from the retina at all times. The subject reacts by giving a name to the color as soon as it impinges upon the field of vision. It is found that in the peripheral and middle regions of the retina the red rays and the green rays are not effective. The subject reacts to them as to non-homogeneous or white light. Only in the central regions of the retina do these rays become effective as monochromatic stimuli. Again, yellow rays and blue rays, when allowed to fall upon the periphery of the eye, are reacted to as white light. But they be- come effective monochromatic light stimuli in the middle regions THE RECEPTORS AND THEIR STIMULI 93 as well as in the central regions of the retina. These various zones are widened with every increase in the energy of the mono- chromatic light. It is possible that with spectral light of suffi- cient intensity such zones would disappear.
Other Phenomena in Monochromatic Light Stimulation. — There are so many other phenomena connected with monochro- matic light vision that we can in the remainder of our discussion do little more than enumerate some of the most important of them. ( 1 ) Unless the intensity of the monochromatic light reaches a certain point, it is reacted to as white light (photochromatic interval). There is one exception to this: There is no photo- chromatic interval for the red rays. In a purified spectrum, that is, where the rays are passed through more than one prism, the rays from 760^ to 647/i/* are effective as soon as the visual limen is crossed, that is, as soon as there is any reaction to light at all. It follows from this that if the intensity of the whole spectrum is reduced to the photochromatic interval, the spectrum as a whole will be reacted to as to white light (the spectrum will be short- ened at the red end). If the relative stimulating effect of each part of such a weakened spectrum is determined, it will be found that the regions offering the greatest stimulating effect differ with the state of adaptation of the eye, if the eye is normal. To the light adapted normal eye the region of highest stimulating effect is approximately 580^ (yellow). To the thoroughly dark adapted eye, it is approximately at 492/^ (blue-green). There is thus a shift in the region which offers the highest stimulating effect due to change in the light adaptation. In the totally color blind, that is, in individuals for whom differences in wave length are not effective stimuli, there is no shift. This shift is some- times spoken of as the Purkinje phenomenon. Since there is relatively little twilight vision in the center of the eye the Pur- kinje effect does not appear in the foveal vision. This shift in stimulating effect is of very great importance in work upon ani- mals, since where it can be obtained it offers good additional evi- dence that wave length can serve as a factor in their adjustments. (2) We saw above that in normal individuals the red and green rays are not reacted to as to monochromatic light but as to 94 PSYCHOLOGY white light in the peripheral and middle* regions of the retina; and further that in the peripheral zone the same defect appears with respect to yellow and blue rays. Occasionally individuals are found, about 3 to 4 out of each 100 males, and not more than 2 out of each 100 females, who show in the central re- gion of the retina one or another form of these anomalies of vision: (a) certain individuals respond to all monochromatic rays as they do to white light. Such individuals have permanent twi- light vision (page 96), and when the relative stimulating effect of the different parts of the retina is determined, its highest point in any state of adaptation is 492/x/x (blue-green), which is the same relation we found to hold true when people with normal vision are tested with dark adapted eyes. Such cases are rare. (6) A much more common form of defect is met with? The indi- vidual responds to red and green rays as he does to white light. His reactions to yellow and blue rays are normal. In such cases the conditions which are found to hold in the case of the middle regions of the retina hold for the central regions as well. Some authors assert that there are two forms of this defect, the pro- teranope and the deuteranope. The difference in behavior of these two types of individuals need not detain us here, (c) Occasionally a rare case is reported in which the responses to the red and green rays are normal but those to the blue and yellow rays are defective.
REACTION TO NON-HOMOGENEOUS LIGHT (" WHITE " LIGHT) Controlling the Stimulus. — One of the most satisfactory ways to obtain white light stimuli is to insert one or more Aubert or iris diaphragms backed with ground glass in the north wall of a dark room. In the dark room the light should fall upon a plaster surface. By opening or closing the diaphragm the intensity of the light may'be varied from threshold to the full capacity of the diaphragm. Instead of using diffuse sunlight, one may illumi- nate the diaphragm by an arc or a tungsten lamp. The intensity of the light also may be altered by the introduction of a rotating sector or by distancing the source light if a lamp is used instead of sunlicrht. One of the commonest ways, of course, of making THE RECEPTORS AND THEIR STIMULI 95 observations upon white light is to use an optical bench. At one end of the bench is placed a standard tungsten lamp and at the other end the variable light which is to be measured. Between the two lights is a photometer. This arrangement is used when- ever quantitative experiments are to be made (differential thresholds, etc.).
Some Phenomena Appearing in the General Use of White Light. — (1) Two white light stimuli have to differ only very slightly in intensity to yield the basis for a differential reaction. When the comparison light differs from the standard by only 17/1000, a difference limen is obtained. (2) The most striking difference is found between the dark adapted eye and the light adapted eye in their absolute thresholds. After an individual has passed from a well lighted room into a dark room, his eye is rela- tively insensitive to white light. The process of progress toward dark adaptation has been studied in detail. If the limen is deter- mined every three minutes, it is found that the intensity of the stimulus necessary to produce a reaction decreases slowly during the first ten minutes, rapidly from the 10th to the 35th minute, slowly again from the 25th to the 65th minute. Adaptation is complete at the end of about 70 minutes. Increase in sensitivity ranges from 1400 to 8000 times. If we call the photometric value of the first threshold 1, then the value of the one obtained at the end of 70 minutes might be approximately 1/8000 of that. These values hold for peripheral regions of the retina, since that be- comes much more sensitive during darkness adaptation. There is relatively little adaptive increase in sensitivity in the center of the retina. When the eye is light adapted, however, as in daylight, the f ovea centralis is the most sensitive area; all reflex movements of the eye tend to bring about the focussing of the object upon the fovea.
The phenomena appearing when the eye is dark adapted are sometimes discussed under the term of "twilight" vision. It is also called rod vision. The following happens: (a) The subject with dark adapted eye reacts to weak monochromatic light stimuli as he does to white light. Differences in wave length do not affect the twilight eye unless the monochromatic 96 PSYCHOLOGY 96 PSYCHOLOGY light stimulation is so strong that it makes the eye become light adapted. (6) We saw also that in a weak spectrum the region offering the highest stimulating value to the dark adapted eye is the blue-green, (c) Non-central regions of the retina become extremely sensitive when the eye is dark adapted, whereas the central region shows little adaptation (it is permanently hemeral- opic).8 (3) Phenomena similar to those already studied with monochromatic light under the heading of after-effects of stimu- lation (negative and positive after-images and simultaneous and successive contrast) appear when non-homogeneous lights are used as stimuli. (4) The retina is sensitive to white light throughout the central, middle and peripheral zones. (5) When non-homogeneous white light and homogeneous light fall together upon a plaster surface, the white light is said to alter the ' ' sat- uration" of the monochromatic light. Indeed, if the white light stimulation is sufficiently intense the subject ceases to react to the combination as a monochromatic light at all. The mono- chromatic light is then said to be ' ' quenched. ' ' Color Theories. — Several general theories of vision have been advanced which have for their purpose the presentation of a pos- sible physiological scheme of the kinds of processes that go on in the eye when light falls upon it. All of the theories assume the presence of some kind of substance that is sensitive to light. One of the theories, that of Mrs. Christine Ladd-Franklin, has several good points. In order to develop this theory, we must mention the fact that only rods are found on the periphery of the retina and that in the periphery all monochromatic lights are reacted to as to white light. In the middle regions of the retina there are both rods and cones. This region is sensitive only to yellow and blue rays and to white light. Finally in the central regions we have sensitivity to all wave lengths and to non-homogeneous 8 There are many pathological cases where adaptation to darkness does not occur even in the non-central regions. Such persons are totally unfitted for work at night. The matter became of some importance during the recent war in selecting scouts for making observations in No Man's Land and for night fliers. It is quite easy to pick out the men who cannot adapt to dark- ness. Whether it is correlated with any particular type of eye is not known.
THE RECEPTORS AND THEIR STIMULI 97 light as well. In order to account for these functional peculiar- ities, Mrs. Franklin assumes (1) a primitive photochemical mole- cule which breaks down under any form of light stimulation. This is found only in the rods. (2) A partially differentiated molecule which is sensitive to yellow and blue rays. This mole- cule is found in the relatively poorly developed cones of the middle zone of the retina. (3) A triply differentiated molecule which breaks down when any one of three monochromatic lights —red rays, green rays and blue rays — impinge upon it. This molecule is found only in the cones in the central regions.9 Hering assumes three pairs of photochemical substances: one pair sensitive to red and green rays, one pair sensitive to yellow and blue rays, and one pair sensitive to non-homogeneous rays. Two antagonistic and simultaneous processes, one kata- bolic, the other anabolic, are constantly taking place in these three pairs of substances. When the red rays strike the eye, the katabolic phase prevails in the red-green substance, and the sub- ject reacts to the red rays. When the green rays strike the eye, the anabolic phase prevails, and the subject reacts to the green rays. Similarly when the yellow rays strike the eye, the kata- bolic phase prevails and the subject reacts to the yellow rays; when the blue rays strike the eye, the anabolic phase prevails, and the subject reacts to the blue rays. Non-homogeneous light as well as monochromatic light at all intensities induce the kata- bolic phase in the white-black substance. These photochemical substances in all color theories are unstable. For example, on the Hering assumption, if the katabolic phase is induced by an appropriate ray, say, 55S/*//, (yellow) striking the eye for a short time, the subject reacts to the katabolic phase as long as the ray impinges upon the eye. As soon as the ray is removed, the anabolic phase begins (there is a constant tendency to establish equilibrium in the photochemical substances) and the subject reacts as though a ray from the blue region of the spectrum 9 Mrs. Franklin developed this theory very early in 1892. Schenck in 1906 advanced almost exactly the same view but with certain modifications. Luciani in his Physiology gives Schenck the credit for advancing the theory. The germ of the theory is of course much older.
7 98 PSYCHOLOGY actually impinged upon the eye. This accounts for the origin of the stimulus in the negative after-image. We saw on page 93 that when certain rays of light are superimposed upon a single surface, the subject reacts not to monochromatic light but to white light. Hering assumes that two rays, the one from the yellow region, the other from the blue region, leave the yellow- blue substance in equilibrium, that is, they balance or cancel each other. But both of these monochromatic lights affect the white-black substance, hence the subject has every reason for reacting to white light, since the appropriate physiological stim- ulus is at hand.10 Most of the phenomena described on page 93 can be explained by one theory as well as another. The Hering theory is criticised for assuming that red rays and green rays cancel each- other.11 The balancing factors for the red rays are found not in the green, but in the blue-green region of the spectrum. It is criticised also for its inability to explain certain types of red-green blind- ness. According to some investigators at least, the ability to react to green rays is sometimes present in cases where the sub- ject cannot react (on the basis of wave length) to red rays. There are possibly many differences in responses between the deuter- anope and the protanope. If sensitivity to red and green rays depends upon reverse effects in a single photochemical substance, loss of response to one should entail loss of response to the other.
STRUCTURE OF THE EYE.
Structure of the Eye as a Whole. — The eye as a whole func- tions somewhat like a camera. In a camera there is a lens for focussing an image upon a sensitive plate, and a diaphragm for controlling the intensity of the light. In the eye there is a lens, and a similar diaphragm, the iris. The sensitive plate of the 10 Attention is called to the fact that the sudden withdrawal of all light from an object serves as a positive stimulus " black." The easiest way to offer this stimulus is to have the subject proceed suddenly from a sunny room to a good dark room.
11 As a matter of fact when red rays of a certain length and green rays of a certain length are superimposed, the subject reacts as 'though stimu- lated by a single monochromatic band of yellow rays.
THE RECEPTORS AND THEIR STIMULI 99 camera becomes the retina of the eye. In the retina must lie the photochemical substances which are assumed to exist. Here the analogy stops. The action of the chemical substances in the retina ends in a neural impulse which passes through the central 9 15 FIG. 16. — Schematic drawing of eye, in cross-section. 1, The optic nerve (really the optic tract); 2, sclerotic coat; 3, choroid coat; 4, the retina; 5, ora serrata, termination of retinal structures; 6, ciliary muscle; 7, iris; 8, cornea; 9, aqueous humor (camera oculi anterior); 10, attachment of choroid to sclerotic forming a fixed point towards which the ciliary muscle draws when contracted; 12, the crystalline lens; 13, tendon of rectus lateralis; 14, vitreous humor (corpus vitreum); 15, fovea centralis.
nervous system and out to an effector organ. Each eye-ball is almost spherical. It is made up of concentric coats, modified in a special way in certain parts, a crystalline lens and two fluid masses — the aqueous and vitreous humors. The lens and fluids occupy the interior of the eye-ball. Fig. 16 shows the more im- portant features of the eye as a whole.
100 PSYCHOLOGY Coats of the Eye.— (1) The outside coat of the eye is a tough fibrous membrane which gives the eye its form. The posterior portion of this coat, the sclerotic, is opaque; the anterior portion, the cornea, is transparent. (2) The middle coat, the choroid, is a black, soft, extensible and easily disrupted membrane. It is composed largely of vascular and muscular tissue. The posterior portion of the choroid is pigmented; the middle portion is muscu- lar (for the control of the lens), while the anterior portion, the iris, is a perforated membranous diaphragm placed just in front of the lens and immediately behind the cornea. The opening in this diaphragm is called the pupil. The iris contains connective tissue fibers arranged radially to the pupil. Near the margin of the pupil there are also smooth muscle fibers which form a sphinc- ter muscle. There are also smooth muscle fibers radially dis- posed that act antagonistically to the sphincter fibers. The size of the pupillary opening is determined by the action of these muscles. Strong light falling upon the retina produces relaxa- tion in the radial fibers and contraction of the sphincters: the pupil grows smaller. When the light is decreased in intensity, or removed, the sphincters relax and the radial fibers contract: the pupil dilates.
The Lens and the Ciliary Muscles. — The crystalline lens (Pig. 16, 12) is biconvex. It is highly elastic and tends always, unless constrained, to become more nearly spherical. It is put under restraint by the transparent capsule which surrounds it. When the eye fixates objects upon the horizon, the lens becomes most flattened. When fixating objects at a distance of 14 inches, it becomes most nearly spherical. The mechanism which brings about the flattening of the lens is the ciliary muscle. The loose muscular coat of the choroid in the region of the ciliary muscle is firmly attached to the firmer sclerotic. The lens capsule is at- tached as shown in Fig. 16. When the eye fixates objects near at hand, the ciliary muscle contracts sharply and as the muscle pulls up towards the point where the choroid is attached to the sclerotic, the tension on the capsule is lessened. The lens, by virtue of its own elasticity, becomes more convex. When the object is far away, the ciliary muscle relaxes, thus dragging THE RECEPTORS AND THEIR STIMULI 101 down the lens capsule, greatly increasing the tension. This exerts uniform pressure principally upon the anterior surface of the lens. The images for near and far objects thus fall accurately upon the retina in the emmetropic or normal eye. In a great many cases the action of the lens and muscle is not accurate. For example, in myopia (near-sightedness) parallel rays of light are brought to a focus in front of the retina; in hypermetropia (long-sightedness) they are brought to a focus behind the retina. These two defects are easily corrected with glasses. Often, too, there is a lack of perfectness in the cornea. It may not have the same radius of curvature in the upper and lower1 halves, or in the right and left halves. The effect on the path of the light is the same as that which would be obtained were the light passed through a lens made up by cementing together halves and quar- ters of lenses of different foci. In such cases of corneal defect no clear, complete image can fall upon the retina. This condi- tion is known as astigmatism.
The Retina. — The retina (Fig. 16, 4) is a delicate.inner mem- brane lining the eye-ball. It is cup-shaped, since it does not invest the anterior portion of the eye-ball. It ends near the ciliary muscle. Nerve cells lying in the retina send out their processes to form the optic tract. This tract pierces the choroid and scle- rotic and takes its exit at the posterior pole of the eye-ball. Fig. 16, 1, gives a view of this relationship. It is usually spoken of as the entrance of the optic nerve. It is really the exit of the optic tract. The relationship of the nerve elements in the optic tract with the other elements in the retina is somewhat compli- cated. Indeed, the whole retina is far from simple. For descrip- tive purposes it is best to start with the actual receptors or sense organs. These are nearest the choroid and hence farthest away from the light. The light must pass through all the other layers here described before it can fall upon the receptors. The receptors are of two kinds, the rods and the cones. They are shown in Fig. 17. The outer segments of both rods and cones are made up of shining, doubly refractive substances, which can be split up by certain reagents into a series of discs. The inner member of both rods and cones is a fine varicose fiber which contains a PSYCHOLOGY FIG. 17,-Diagrammatic thT!?S,£e ^h?
nucleus. The imier segments of the rods terminate in a rounded swell- ing, whereas the cone terminates in a swelling which splits up into fine processes. It can be seen that the cone is probably the more highly complex of the two structures, since by its branched ending it offers more chances for neural connections. We have already brought out the fact that in the fovea centralis, or spot of clearest vision, only cones are present. In the regions around the fovea there is a one-to-one relation- ship between rods and cones. As one passes from this region, one finds fewer and fewer cones. On the periphery only rods are present. The rod and cone layer may be called the outermost functional layer of the retina (that is, it lies next to the sketch choroid). As in all other sense organs, the receiving structures must come in contact with neural struc- tures. The rods and cones form no exception. The endsi of their inner dr; T,enubcYeusf of 'cone* processes come into contact with the L°off rodland second functional layer of the retina, the layer of bipolar neurones (a neurone is a nerve cell with its outgrowth) (Fig. 17). This layer may be looked upon as the true peripheral ^ optic nerve, although the neurones composing it are only a fraction of a mm. in length. The cell bodies in this layer give off at one pole an outer process which ends around the terminal processes of the inner structures of the rods and cones. At its other pole, it gives THE RECEPTORS AND THEIR STIMULI 103 off a process which ends around the cell bodies that lie on the inner surface of the retina (next to the vitreous humor). These cell bodies with their processes should be looked upon as the third functional layer of the retina. This innermost layer consists of fairly large nerve cells, each of which gives off a single fiber which passes back to the pole of the eye. This fiber, regardless of the position of the cell body giving rise to it, takes a curved radial course and joins with all fibers of similar origin to form the optic tract. Their further course to the brain is described on page 147.