Human Vision
We all take our vision for granted and it is only when we start to get problems that we begin to think about it. Following our March lecture by Professor Jim Bowmaker of the Institute of Ophthalmology, University College London, everyone was thinking about vision and how complex it really is.
We have vision to find food, find a mate and finally not to be food for something else. The eye is essentially a camera. It is made up of five types of cells and the retina (the light sensitive part) has two types of receptors, the cones and the rods. The cones are used in daylight and give us colour vision. There are a total of about six million cones in our eye. The rods are used in low light. They are more concentrated towards the edge of the retina and so astronomers often observe faint objects with slightly averted vision in order to better use them. The rods give no colour, which is why we only see faint objects through a telescope in black and white. There are about 120 million rods in the average eye.
The cones have high acuity, but low sensitivity. It takes at minimum the light of a bright moon to start the cones working. Rods on the other hand have low acuity, but high sensitivity in the blue-green spectrum and will work in near darkness. There is an overlap in light level where the cones and rods work together. Astronomers often speak of dark adaptation. This happens when you walk from a light room into darkness and at first you can see nothing. Cones adapt to the dark in 10 to 15 minutes, whilst the rods take about 35 minutes. This is why astronomers only use red torches as the red light keeps the cones active but the rods inactive. Shining a torch into the astronomers face can destroy their dark adaptation in less than a second. It will then take another 35 minutes to get adapted again. This is one reason astronomers campaign against incorrect streetlights and the dreaded security light.
In the eye, the rod membrane is always growing and the excess is 'bitten' off when we awaken in the morning. If you have sore eyes in the morning then this could be what has happened! The eye absorbs this particle of rod and it doesn't cause any harm.
Humans and apes are trichromats as we see light in three colour channels: Red, Green and Blue. Fish and birds are tetrachromats, meaning they can see four colour channels. In other words, your goldfish or budgie can see colour better than you!
Our talk ended with images that are used to test people for colour blindness, and images which had been adapted to show us what we would see if we were colour blind.
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