Stephen Cockrill, age 12, of ScottsdaIe, Ariz., for his question:
What makes a cats eyes shine in the dark?
The happiest cat is one with both a home and freedom to roam, As a rule, she is a loving pet and during the day she will romp and plat, especially with young children. From time to time she takes a nap, which is in preparation for her night time prowl. When the house is fast asleep, Miss Puss slips outdoors, all set for a round of adventure.
She may get into a fight and wake the neighbors with banshee howls. But the main thing on her mind is dessert. She is, of course, a meat eater and she enjoys the fish, liver and hamburger you give her as a main course. But her idea of dessert is a fat juicy mouse. And pretty puss is well equipped to catch herself one, two or three of these before the night is done.
She moves on padded paws, soft as a feather. She has endless patience to wait and watch. She has a fine sense of smell and she can see by the dimmest of starlight. For this, she has special eyes which shine in the dim light. No, puss cannot see and nor do her eyes shine in total darkness. Her wonderful eyes are able to use a double scoop of dim light and, in the process, some of the light is reflected back as eyeshine.
Your eye, of course, has a round pupil. In a bright light, your pupil contracts to shut out some of the glare. Puss has an almond shaped slit pupil and in a bright light the slit becomes narrower for the same reason. In the dim lights your pupil opens wide to catch as much light as possible. Puss's pupil becomes almost as big as her entire eye for the same reason, But she has still another trick to catch the light. .
Light passes through out round pupil to the retina in the book of your eye. This is a dark screen which absorbs the light and the image of the scenery it carries.
Complex nerves then flash these impressions to the brain. Miss Puss has a layer of glassy cells behind her retina and it acts like a reflectorbutton. At night, the dim light passes through her wide open pupil to the retina, then through to the glassy layer behind the retina. But here it reflects or bounces back.
In this way, the cat's eye gets a double exposure of the scenery. Dark areas appear darker and light areas appear lighter. This is why Miss puss can see better in a dim light than we can: She can certainly see better than a mouse, whose vision is not too good at the best of times, But, for Miss Puss, there is a catch to this double exposure trick.
Light passes through the wide open slit pupil to the retina where impressions are then assembled through to the glassy layer behind the retina. From here it bounces back through the retina again, giving a doubleset of light and dark impressions, Some of this light, however, goes straight forward and comes out through the pupil. This is the eyeshine which glows in the semi darkness. This eyeshine does not help the cat see in a dim light. Instead, it may hinder her by flashing an eerie warning to her victims,