Tag Archives: Photoreceptors

Eureka

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It is a truth universally acknowledged that Galileo was a bit of a clever geezer and what troubled him should rightly concern us. Of course, he compounded his problems by developing the telescope so he opened his senses up to things that had never troubled Homo sapiens before. One thing that kept him awake at night, apart from wondering when the Inquisition would next pop up, was why the planet Venus appeared bigger than Jupiter to the naked eye but when viewed through a telescope the reverse was true. I assume he was holding the telescope the right way round at the time. Seemingly, this is a problem that has baffled scientists for more than 400 years.

But no more. When viewed directly with the naked eye Venus appears to have a radiant crown which makes it look eight to ten times bigger than Jupiter, even though Jupiter is four times larger when seen from planet Earth. Galileo thought that this radiant crown was something to do with what he called an impediment of our eyes which was eliminated when we use a telescope. He thought the phenomenon was due to optical interference to the light of the planets as it entered our eyes.

Researchers, led by Jens Kremkov at the State University of New York College of Optometry – the results were published in the Proceedings of the National Academy of Science – have shed new light on to the problem. The effect, it seems, is down to the way the light-sensitive cells at the back of the eye respond to images of different intensity set against a dark background. The light coming from Venus is brighter than that from Jupiter and the retina and brain are finely tuned to respond to the contrast between light objects against a dark background. The brighter light of the smaller plant makes it seem larger than the duller larger planet.

To test their theory the scientists used electrodes to record the electrical signals from neurons in the visual areas of anaesthetised cats, monkeys and human brains whilst the animals and humans were shown dark shapes on a light background, light shapes on a dark background and light and dark shapes on a grey background. They found that white spots on a black background looked bigger than same-sized black spots on a white background. The source of Galileo’s problem is the way neurons are laid out and inter-connected in the retina and brain and, the researchers surmise, it has ever been thus since the development of sight in the photoreceptors of the eye.

The explanation to Galileo’s problem also helps explain why we are more comfortable reading black print on white than white text on black – a fact that users of more adventurous blog templates would do well to remember.

Isn’t science wonderful!