I. Introduction
The mysteries of color vision have long fascinated humanity. Are the colors we see the same as the shades others perceive? What about animals? And, the most fundamental question of all, how does the eye see color?
The earliest theory of color vision was proposed in 1790 by Thomas Young, an English scientist and polymath. Young suggested that the human eye has the ability to see only three colors (red, blue, and yellow), and that all other colors are a combination of these three hues. While Young wasn’t entirely accurate, his discoveries provided a solid foundation for scientists to further develop. In the mid-1800s, Max Schultze, a German microscope anatomist, expounded upon Young’s hypothesis by discovering that the eye contains certain cells that show sensitivity to color.
As Dr. Patel described, the retina is the nerve that lines the back of the eye. When light enters the eye through the iris (color is, in essence, nothing more than light that bounces off objects and into our eyes), it stimulates the retina and sends a signal through the optic nerve to the brain. Through a complex series of maneuvers, the brain is able to determine what color the eye is seeing.
There are two types of cells located in the retina: rod cells and cone cells. Rod cells are sensitive to light—a single photon can stimulate a response in a rod cell. Cone cells don’t react to light, but they are extremely sensitive to different colors. There are three types of cone cells, each one containing a different pigment (natural coloring for tissues). These pigments consist of red, blue, and green—close to the colors mentioned in Thomas Young’s original color vision theory. The cone cells are able to distinguish red, blue, and green, and the brain mixes these colors in infinite combinations so we perceive an enormous range of colors.
Max Schultze, a German scientist, discovered the rods and cones by studying the eyes of birds. He noted that nocturnal birds had different retinal cells than birds that were active during the day, and assumed that the nocturnal birds’ eyes were designed to see in the dark. The rod-shaped cells in their retinas, he concluded, must enable eyes to be more sensitive to light. Similarly, the cone-shaped cells in the eyes of “daytime” birds must allow them to see color and fine detail, making it easier to pick out animals on the ground when hunting. Schultze applied the same principle to human eyes: rod cells are responsible for light, cone cells handle color.
III. Biography of Investigator: Max Schultze
Max Schultze (1825-1874) was a German zoologist and microscope anatomist. Like his father, who was the professor of anatomy at the University of Greifswald in Greifswald, Germany, Schultze’s passion in life was biology. He spent his career analyzing the cellular structure of a wide variety of animals, and one of his greatest discoveries was that the cells of every organisms are made up of protoplasm and contain a nucleus. In addition to cellular structure, Schultze studied protozoa, sense organs, muscles, and nerve endings. In 1866, he formed what is known as the “duplicity theory of vision,” in which he studied the retinal cells of birds to discover how eyes are able to perceive color. Schultze served as the professor of zoology at the University of Bonn in Bonn, Germany, until his sudden death in 1874 from a perforated ulcer.
Schultze’s discovery of cone and rod cells led to several other scientific theories regarding how color vision operates in animals other than humans. As Schultze confirmed, humans have three sets of cones for detecting color in different wavelengths—this is called trichromacy. Some animals, however, have only two sets of cones (dichromatic). Because of this, scientists deduced that animals with two types of cones are color-blind to specific colors. Dogs, for example, lack the ability to differentiate between orange and green. Cats, too, possess only two varieties of cone cells; they can’t distinguish colors in the red family. To make up for this disadvantage, the retinas of these animals contain more rod cells than humans, allowing them greater night vision and a stronger ability to sense motion.
Unlike dogs and cats, sea mammals (including sea lions, dolphins, and whales) have a single type of cone designed for viewing patterns in light. However, the cone is unable to detect color at all. These animals are believed to be totally color-blind. In contrast, honeybees and butterflies have four pigments, and are able to see ultra-violet colors that are invisible to humans.
In addition to animal vision, the discovery of rods and cones have made it possible for scientists to investigate color-blindness in humans. Some people (usually males) are born without a certain type of cone cell, or with mutated cones. When this occurs, the person’s vision may be dichromatic. Total color-blindness (seeing the world in black and white) is very rare, but it can occur if only one type of cone cell exists in an individual’s retina.
Impulses from the rods and cones are diffused across the retina via nerve fibers. The fibers then merge together and form the optic nerve. The left optic nerve and right optic nerve (one from each eye) connect at a point known as the optic chiasma; from there, each nerve divides into two branches. The inner division from each eye crosses over and joins the outer branch from the other eye. Because of this, the impulses from the side of each eye arrive in the cerebral cortex of the brain; the impulses from the right half of each eye end up in the right cerebral cortex. Because the light rays entering the eye are slightly rearranged when they cross each other, the image formed in the retina is upside-down. Using a method unknown to modern scientists, the brain is able to flip the image right-side up, as well as fuses the “left eye image” and :right eye image” together to form a single image.
VI. Works Cited
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