Section 36–2 The physiology of the eye

(Underwater vision / Binocular vision / Inverted vision)

 

In this section, Feynman discusses neural machinery, optical components, and neural connections of human eyes. However, the discussions could be related to underwater vision, binocular vision, and inverted vision.

 

1. Underwater vision:

“This is why we cannot see clearly under water, because we then do not have enough difference between the index of the cornea, which is 1.37, and that of the water, which is 1.33. Behind the cornea is water, practically, with an index of 1.33, and behind that is a lens which has a very interesting structure: it is a series of layers, like an onion, except that it is all transparent, and it has an index of 1.40 in the middle and 1.38 at the outside (Feynman et al., 1963, p. 36–3).”

 

Feynman explains that we cannot see clearly under water because the difference between the refractive index of the cornea and water is not enough. One may clarify the ability of cornea, humor, and lens of the human eye to focus images (bend light rays) on the retina, but the clarity of the images is dependent on the ratio of refractive index of the cornea and that of the medium where the object resides. Based on research data over the recent 20 years, the refractive index of cornea changes from about 1.400 at the epithelium to 1.373 at the endothelium (Patel & Tutchenko, 2019). However, the unclear underwater vision is also due to the light absorption in water whereby the attenuation is dependent on the wavelength of light. Furthermore, the absorption and attenuation are dependent on the depth and turbidity of the water.

 

It should be worth mentioning that the human eye is adapted for viewing in air because the ratio of the refractive index of cornea to that of the air (Snell’s law of refraction) is relatively larger. On the other hand, fishes have a better underwater vision because their corneas are more spherical and thus, they can bend light rays better to achieve a better focus. A disadvantage of fishes’ corneas is that it causes them to be short-sighted when viewing in air. Interestingly, penguins are able to focus more clearly on land and underwater, but their corneas are relatively flatter than other birds (Hadden & Zhang, 2023). Essentially, the penguins’ lenses are softer and their eye muscles can adjust the shape of the lens more than other animals to view in air and underwater.

 

2. Binocular vision:

“This is the manner in which the information from each of the two eyes is put together in order to tell how far away things are. This is the system of binocular vision (Feynman et al., 1963, p. 36–4).”

 

Binocular vision is the integration of monocular sensory and motor visual information into a combined percept of the surrounding physical space (Stidwill & Fletcher, 2017). There are at least three conditions for binocular vision: (1) Interocular distance: Two eyes and a separation between the eyes (interocular distance), which is about 65 mm in adult humans. (2) Neural pathway: A neural pathway (or neural connections) to transfer the two images to the brain. (3) Neural machinery: Neural processing systems to integrate the different types of visual information, such as luminosity, size, color and contrast. According to Feynman, the binocular vision tells how far away things are, but one may add that it is the ability of human eyes to perceive a three-dimensional image by combining the two images from the left and right eyes simultaneously.

 

“… it is impossible consciously or unconsciously to turn both eyes out at the same time, not because there are no muscles, but because there is no way to send a signal to turn both eyes out, unless we have had an accident or there is something the matter, for instance if a nerve has been cut. Although the muscles of one eye can certainly steer that eye about, not even a Yogi is able to move both eyes out freely under voluntary control, because there does not seem to be any way to do it (Feynman et al., 1963, p. 36–5).”

 

There are two aspects of binocular vision, convergent vision and divergent vision, which is the ability of both eyes to move together to perceive depth and distance. In general, vergence eye movements consist of simultaneous movement of both eyes in opposite directions that maintain binocular vision. Convergent vision (or convergence eye movement) occurs when both eyes focus on a nearby object, i.e., the eyes rotate inward, towards each other, to align their lines of sight onto the object. On the contrary, divergent vision (or divergence eye movement) occurs when both eyes focus on a distant object, whereby the eyes rotate outward, away from each other, to align their lines of sight onto the object. Interestingly, a person (non-Yoji) may have partial divergence eye movement, e.g., right eye diverging while left eye remains relatively stable as shown below.

 

Source: Vergence - Wikipedia

3. Inverted vision:

“Believe it or not, the up-and-down line in our visual field as we look at something is of such a nature that the information from all the points on the right side of that line is going into the left side of the brain, and information from the points on the left side is going into the right side of the brain, and the way this area is made, there is a cut right down through the middle, so that the things that are very close together right in the middle are very far apart in the brain! (Feynman et al., 1963, p. 36–4).”

 

One may adopt the term inverted vision (or left-right reversed vision) that refers to a vision related to the interpretations of the eye-brain system. In short, the inverted image formed on the retina of the eye is “re-inverted” by the brain. In other words, the visual information is processed and inverted by the brain to present a coherent and upright perception of the world observed by the eyes. On the contrary, a lady by the name of Marietta Everett had up-down inverted vision and see the world upside down (Davis, 1969). Due to the image is inverted on the retina and abnormalities in the visual cortex, the brain would receive a reversed representation of the visual scene. While most people perceive the world upright through perceptual adaptation, Everett’s brain was unable to perform this adaptation, resulting in her perception of the world as inverted.

“This is an important point, because most of the earlier books on anatomy and psychology, and so on, do not appreciate or do not emphasize the fact that we are so completely wired already—they say that everything is just learned (Feynman et al., 1963, p. 36–6).”

 

Perceptual adaptation refers to the brain’s ability to adapt to distorted sensory inputs over time, allowing individuals to perceive these inputs as normal or familiar. In Erismann and Kohler’s Innsbruck Goggle experiments, perceptual adaptation is demonstrated through the subjects’ ability to adapt to wearing upside down goggles (e.g., flipping top and bottom or flipping left and right). Despite initially perceiving a distorted world through the goggles, over time the subjects were able to interpret the transposed visual information as normal. In other words, the perceptual experience of the reversed image may become normalized, and individuals may perceive it as if it were in its original orientation. The orientation of visual information in the brain is determined by the arrangement of neural connections from the eyes to the visual cortex.

 

Review Questions:

1. How would you explain underwater vision?

2. How would you explain binocular vision?

3. How would you explain inverted vision?

 

The moral of the lesson: Underwater vision, binocular vision, and inverted vision are related to refractive index, eye movements (or ocular motility), and perceptual adaptation.

 

References:

1. Davis, B. (1969). Upside-down girl baffles scientist, Jet, 36(26), 17-22.

2. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley.

3. Hadden, P. W., & Zhang, J. (2023). An overview of the penguin visual system. Vision, 7(1), 6.

4. Patel, S., & Tutchenko, L. (2019). The refractive index of the human cornea: A review. Contact Lens and Anterior Eye, 42(5), 575-580.

5. Stidwill, D., & Fletcher, R. (2017). Normal binocular vision: Theory, investigation and practical aspects. John Wiley & Sons.

Section 36–1 The sensation of color

 (Land color effect / Benham’s Disk / Opponent-process theory)

 

In this section, Feynman discusses Land color effect, Benham’s Disk (Fechner colors), and opponent-process theory of color vision that are related to subjective sensation of colors. For example, Fechner colors could be defined as the creation of illusory (subjective) colors through a repeated pattern of black and white stimulus. Thus, the section could be specified as “the subjective sensation of color.”

 

1. Land color effect:

“In fact, Land has shown that if we mix that apparent blue and the red in various proportions, by using two photographic transparencies with absorption in front of the red and the white in different proportions, it can be made to represent a real scene, with real objects, rather faithfully. In this case we get a lot of intermediate apparent colors too, analogous to what we would get by mixing red and blue-green; it seems to be an almost complete set of colors, but if we look very hard at them, they are not so very good. Even so, it is surprising how much can be obtained from just red and white (Feynman et al., 1963, p. 36–1).”

 

Edwin Land’s two-color experiment involving white light and red light was a pioneering experiment in color vision. Specifically, Land was using two black-and-white transparencies of the same colored-scene: the first transparency was taken through a red filter and the second transparency was taken through a green filter (See figure below). In other words, one may emphasize the need of illuminating a photograph (long record) with longer wavelengths of light and the photograph (short record) with shorter wavelengths of light (Land, 1959). The experiment demonstrates that, through the mixing of red and white light, observers could perceive a variety of intermediate colors, creating the illusion of a broad color spectrum. Importantly, Land’s experiment could not be explained by classical color theory principles, which need three primary colors to form a wide gamut of colors.

 

Source: Land, 1959

Note: Edwin H. Land was the inventor of instant photography or Polaroid camera. According to some writers, e.g., Erik Calonius, Steve Jobs was inspired by Land. In his biography of Steve Jobs, Isaacson (2011) quotes Jobs as saying that Land was one of his heroes.

 

Edwin Land proposed the retinex theory of color vision, which requires the brain’s interpretation of relative differences in light reflected from surfaces instead of the intensity of light that reaches the eyes. The word “retinex” is a mix of “retina” and “cortex,” meaning that both the eye and the brain are involved in the sensing of color. The theory was also used to explain the phenomenon of color constancy under a wide range of lighting conditions. In actual scenes, the perceived color of each region depends on the colors of its neighboring points and the overall color of the scene. Essentially, Land’s experiment seek to understand whether color is inherent in the physical world or it is a subjective sensation of human eye.

 

2. Benham's Disk:

“Another example is the appearance of “colors” in a black-and-white rotating disc, whose black and white areas are as shown in Fig. 36–1. When the disc is rotated, the variations of light and dark at any one radius are exactly the same; it is only the background that is different for the two kinds of “stripes.” Yet one of the “rings” appears colored with one color and the other with another (Feynman et al., 1963, p. 36–1).”

 

At the end of the previous lecture, Feynman discussed the phenomenon of Fechner colors*. The color effect is also known as the Benham’s Top or Benham's Disk, which involves the sensation of colors when an object with black-and-white pattern is rotated. The sensation of colors depends on several factors, such as the speed of rotation, i.e., the brain interprets colors due to the integration and motion of the black-and-white pattern. For example, if the disk (as shown below) is rotated clockwise, the innermost arcs may form red rings, the inner arcs may form orange rings, the outer arcs may form green rings, and the outermost arcs may form blue rings. If it is rotated anticlockwise, the pattern is reversed, with the innermost arcs form blue rings and the outermost red rings. Brighter light conditions may result in more vivid or saturated colors, while dimmer light may lead to less distinct colors or grayscale appearance.

Source: Nishiyama, 2014.

 

*In the Audio Recordings of the previous lecture, Feynman says something like: “It is comparing what it sees in one region with another not in the conscious way, but already in retinal level and this is demonstrated by this crazy color phenomenon known as the Fechner colors.”

 

Source: The Feynman Lectures Audio Collection: https://www.feynmanlectures.caltech.edu/flptapes.html

 

“No one yet understands the reason for those colors, but it is clear that information is being put together at a very elementary level, in the eye itself, most likely (Feynman et al., 1963, p. 36–1).”

 

The Pattern Induced Flicker Colors (PIFC) model and Yutaka Nishiyama’s Dynamic Interference model, e.g., offer explanations for the phenomenon observed in Benham's disk, but they approach it from different perspectives. Firstly, the PIFC model suggests that the sensation of colors in Benham’s disk arises from interactions between different types of retinal cells and neural circuits. It proposes that the alternating black and white patterns on the disk stimulate different types of retinal cells differently, leading to the sensation of colors. On the other hand, the interference model attributes the perception of colors in Benham’s disk to interference patterns generated within the visual system. It suggests that the black and white patterns on the disk create interference patterns as they move across the retina. Both models offer insights into the complex processes underlying the sensation of colors in Benham’s disk.

 

3. Opponent process theory:

“The fact that there are three pigments does not mean that there must be three kinds of sensations. One of the other theories of color vision has it that there are really opposing color schemes. That is, one of the nerve fibers carries a lot of impulses if there is yellow being seen, and less than usual for blue. Another nerve fiber carries green and red information in the same way, and another, white and black (Feynman et al., 1963, p. 36–2).”

 

Perhaps Feynman could have used the term “opponent process theory of color vision” instead of opposing color schemes. Historically, Ewald Hering, a German physiologist, made significant contributions to this color theory in the late 19th century. Hering suggested that color vision involves three sets of receptors (red-green, blue-yellow, black-white) where these pairs of colors cannot be perceived simultaneously. (For example, a cell was excited by green light would be inhibited by red light, and vice versa.) It was proposed to understand the observation of color afterimages, where staring at one color for an extended period leads to the perception of its complementary color after looking away. It implies that we cannot sense greenish reds or yellowish-blues as colors. In a sense, the theory was based on Hering’s belief that Helmholtz was unable to give a good physiological explanation for color-contrasted phenomena (Finger, 2001). 

 

“In the modern literature all we find on the subject are repeats of the same statement, or of one by a German psychologist, who uses as one of his authorities Leonardo da Vinci, who, of course, we all know was a great artist. He says, ‘Leonardo thought there were five colors’ (Feynman et al., 1963, p. 36–2).”

 

Perhaps Feynman could have identified the German psychologist that says, “Leonardo thought there were five colors.” However, Leonardo da Vinci writes, “White is given by light, without which no color may be seen, yellow by earth, green by water, blue by air and red by fire, and black by darkness which stands above the element of fire, because there is no substance or dimension on which the rays are able to percuss and accordingly to illuminate it (cited in Hoeppe, 2007, p. 59).” In short, da Vinci thought there were six colors: white, yellow (earth), green (water), blue (air), red (fire) and black. However, da Vinci was inconsistent and sometimes did not include black and white as colors (Kuehni & Schwarz, 2008). 

 

Review Questions:

1. How would you explain the Land color effect?

2. How would you explain the Fechner colors?

3. How would you explain the opponent process theory of color vision?

 

The moral of the lesson: Color is not merely a property of light or objects—it is a construction of the mind. These phenomena reveal that our perception of color arises not from passive reception of wavelengths, but from the brain’s active interpretation of contrasts, context, timing, and neural opponency. Whether through Land’s two-color illuminations, Benham’s rotating patterns, or Hering’s opposing channels, we are reminded that what we “see” is a subjective reality—a careful negotiation between the physical world and the perceptual machinery of eye and brain. Thus, in both science and daily experience, we must remain humble: our senses do not simply mirror the world, but is an act of interpretation and meaning-making. 

References:

1. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley.

2. Finger, S. (2001). Origins of neuroscience: a history of explorations into brain function. Oxford University Press.

3. Hoeppe, G. (2007). Why the sky is blue: discovering the color of life. Princeton, New Jersey: Princeton University Press.

4. Kuehni, R. G., & Schwarz, A. (2008). Color ordered: a survey of color systems from antiquity to the present. New York: Oxford University Press.

5. Land, E. H. (1959). Experiments in color vision. Scientific American, 200(5), 84-99.

6. Isaacson, W. (2011). Steve Jobs. New York: Simon and Schuster.

7. Nishiyama, Y. (2014). The Mathematics of Benham’s Top. International Journal of Pure and Applied Mathematics, 93(3), 399.

35–6 Physiochemistry of color vision

 (Rod visual pigment / Cone visual pigment / Color sensation)

 

In this section, Feynman discusses the rod visual pigment, cone visual pigment, and ends the chapter by demonstrating the subjectivity of color sensation. The two demonstrations are sometimes known as the Land effect and Fechner color effect.

 

1. Rod visual pigment:

“The pigments that can be obtained from a retina consist mainly of a pigment called visual purple... This fact was discovered in 1877 (Feynman et al., 1963, p. 35–9).”

 

Feynman adopts the archaic term visual purple instead of rhodopsin and mentions that the fact was discovered in 1877. Historically, the discovery of rhodopsin are attributed to two German physiologists Franz Christian Boll and Wilhelm Kühne. In Jan 1876, Boll described the color of rods dissected from dark adapted frog retinas as “Sehrot” (Wade, 2008). Kühne coined the term rhodopsin, in which “rhodo” and “opsis” refer to rose and sight respectively. Furthermore, Boll’s “Sehrot” was translated as vision red and Kühne's “Sehpurpur” as vision purple. Instead of saying a retina consists mainly of a pigment called visual purple, one may clarify that the numerical difference between rods and cones in the retina is substantial: about 120 million rod cells and 6 to 7 million cone cells (having different visual pigments). Remarkably, Feynman suggests that this is due to the need of more visual purple to see at low lighting conditions.

 

Note: In section 36–3 The rod cells, Feynman clarifies that “[t]here are layer after layer of plane structures, shown magnified at the right, which contain the substance rhodopsin (visual purple), the dye, or pigment, which produces the effects of vision in the rods. The rhodopsin, which is the pigment, is a big protein which contains a special group called retinene, which can be taken off the protein, and which is, undoubtedly, the main cause of the absorption of light.”

 

“The most remarkable features of this are, first, that it is in the eye of almost every vertebrate animal, and second, that its response curve fits beautifully with the sensitivity of the eye, as seen in Fig. 35–9, in which are plotted on the same scale the absorption of visual purple and the sensitivity of the dark-adapted eye... (Feynman et al., 1963, p. 35–9).”

 

Feynman explains that the response curve fits beautifully with the sensitivity of the eye in Fig. 35–9, but it oversimplifies their relationship. However, most researchers in the field of color vision had produced widely differing sensitivity curves for the mechanisms of the trichromatic theory (Davson, 1962). The dark-adapted sensitivity curve is influenced by the combined responses of rod cells containing rhodopsin, and the relationship is more complex than the beautiful fit as shown in the graph. On the other hand, a remarkable feature of the rhodopsin is composed of a protein called opsin and a light-sensitive molecule known as retinal, which is derived from vitamin A. Ensuring an adequate intake of vitamin A through a balanced diet is crucial for preventing night blindness. George Wald, a New Yorker, was awarded the 1967 Nobel Prize in Physiology or Medicine for his discoveries about chemical and physiological visual processes in the eye (including vitamin A is found in rhodopsin).

 

Note: In the next chapter, Feynman mentions that if we do not eat enough of Vitamin A, we do not get a supply of retinene, and the eye becomes what we call night blind. (see section 36-3)

 

2. Cone visual pigments:

“The light goes right down into the sensitive point, bounces at the bottom and comes back out again, having traversed a considerable amount of the color-vision pigment; also, by looking at the fovea, where there are no rods, one is not confused by visual purple. But the color of the retina has been seen a long time ago: it is a sort of orangey pink; then there are all the blood vessels, and the color of the material at the back, and so on (Feynman et al., 1963, p. 35–9).”

 

Feynman says that the color of the retina is a sort of orangey pink, however, the light goes right down into the sensitive point, and it traverses a considerable amount of the color-vision pigment (or cone visual pigments). Furthermore, the description of the color of the retina as orangey pink oversimplifies the complex composition of the retina, i.e., the retina does not have a uniform color. When we look at the retina using an ophthalmoscope (see figure below), we can see an orange background, also known as the fundus, with a network of arteries (Valberg, 2007). The fundus refers to the interior surface of the eye, including the retina, optic disc, and blood vessels. In the context of an eye examination, it is more appropriate to describe the “color of the fundus” rather than the “color of the retina.” The orange color of the normal fundus is a result of complex interactions among the visual pigments, vascularization, and the optical properties of the eye.

 

Source: Fundus photography - Wikipedia

“How do we know when we are looking at the pigment? Answer: First we take a color-blind person, who has fewer pigments and for whom it is therefore easier to make the analysis. Second, the various pigments, like visual purple, have an intensity change when they are bleached by light; when we shine light on them they change their concentration (Feynman et al., 1963, p. 35–9).”

 

Bleaching occurs in both rod and cone visual pigments, although the process may differ slightly between them. Perhaps Feynman could have clarified that the term bleaching in this context whether it simply means a change in color to white, almost transparent, or other colors with a deeper meaning. In early 1876, Boll noted that the frog retina is paler after light exposure and can become completely colorless in direct sunlight. On the other hand, Kühne established the notion of “visual cycle”, i.e., visual purple in the rods is bleached by light to form visual yellow which is later transformed into visual white (Wade, 2008). Kühne also showed that the rate of bleaching was dependent not only on the intensity of light but also on its wavelength.

 

In his Nobel Lecture, Wald (1968) explains: “We have been in the habit of saying that light bleaches visual pigments. What it does however is to isomerize the chromophore. The end of this process, if it is allowed to go to completion, is a steady-state mixture of isomers of the chromophore, in proportions that depend upon the wavelength of irradiation and the relative quantum efficiencies of the photoreactions.” In short, bleaching refers to the photodecomposition of the pigment molecules triggered by light absorption.

 

3. Color sensation:

“Color is not a question of the physics of the light itself. Color is a sensation, and the sensation for different colors is different in different circumstances (Feynman et al., 1963, p. 35–10).”

Colors are a result of the complex interaction between light, objects, and the human visual system. The concept of color can be defined from the perspective of cone cells, visual spectrum, and human observer: (1) Cone cells: Color perception is subjective and varies from person to person, i.e., influenced by individual differences in the sensitivity of the eye's cones; (2) Color is the perceptual result of light incident upon the retina in the visible region of the spectrum, having wavelengths in the region of 400 nm to 700 nm; (3) Color is a product of human perception, and it doesn’t exist in the same way without an observer with the ability to perceive and interpret the visual stimuli. To show that color is a sensation and not simply the effect of physical light, Feynman ends the lecture by demonstrating the Land effect and Fechner color effect.

 

In the Audio Recordings* [57 min: 00 sec] of this lecture, Feynman says something like: “One of the possible explanations is this: that the three different color receptors have different timing of response and so because of the flashings, the red and green and so on information come to the head at different times and so you get different effective colors. But look if in this region here, the eye if it held in one spot would see black, black, black, and white and in this region black, black, black, and white, and so on the same proportion. In other words, in coming to the eye here and here are the same flashings exactly (three parts black and one part white) and nevertheless the colors are different. Why? The only difference in the two cases is the characteristics of the background and neighborhood of the bar and therefore there is an integration of the information at the background and the bar (not just the bar). It is very important to appreciate that the retina is already thinking about the light. It is comparing what it sees in one region with another not in the conscious way, but already in retinal level and this is demonstrated by this crazy color phenomenon known as the Fechner colors. Thank you very much.” After the lecture, one undergraduate approached Feynman and mentioned that he had read about Land’s theory some time ago, however, the Land effect is elaborated in the beginning of the next lecture.

*The Feynman Lectures Audio Collection: https://www.feynmanlectures.caltech.edu/flptapes.html

 

Review Questions:

1. How would you explain the most remarkable features of rod visual pigments?

2. What is the meaning of bleaching in the context of cone visual pigments?

3. How would you explain the sensation for different colors is different in different circumstances?

 

The moral of the lesson: The absorption curve of visual pigments in the eye aligns well with the sensitivity curve of the dark adapted eye, based on Rushton’s ophthalmoscope method to detect changes in pigment concentration, which provide insights on visual pigments and color perception.

 

References:

1. Davson, H. (Ed.). (2014). The Visual Process: The Eye. Academic Press.

2. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley.

3. Valberg, A. (2007). Light vision color. Hoboken, NJ: John Wiley & Sons.

4. Wade, N. J. (2008). Visual purple (Sehpurpur). Perception, 37(11), 1617-1620.

5. Wald, G. (1968). Molecular basis of visual excitation. Science, 162(3850), 230-239.