英语时差：猫是“色盲”（上）

You can look at that quilt your cat is curled up on and see a patchwork of greens, blues, and yellows. Your cat, however, cannot see color variation in the quilt.And here's Why? Both humans and cats depend on the back layer of the eye, which is called the retina, to sense light and to distinguish between colors. The retina is made up, in part, of nerve cells that convert color and light into messages they send to the brain. There are two types of cells: rods and cones. Rods are very sensitive to light, though they are not used in color vision. Cones on the other hand are not very sensitive to light but can be used in color vision. Because cats have more rods than humans, they use these light-sensitive cells to see in even very dim light. Likewise, because cats have fewer cones than humans, they cannot distinguish between colors the way humans can.