Page 15 - Demo

Talking glaucoma 15Eye pressure and visual field tests are the most common ways to check how glaucoma is developing and how well treatment is working. But the results can be mystifying, not least because they use strange units: millimetres of mercury and decibels.What%u2019s mercury got to do with eye pressure?One of the most important measurements for people with glaucoma is eye pressure. Keep that under control and you can probably protect your vision.The engineers among you, or those who remember their school physics lessons, might remember that pressure is normally measured in pascals. But your eye pressure results are always given in a weird unit: mmHg, or millimetres of mercury. Why?This apparently strange unit dates back to the 17th century when the Italian physicist Torricelli invented the mercury barometer to measure the pressure of the air. His barometer consisted of a long vertical tube, closed at the top, containing mercury. The bottom of the tube was open, resting in a reservoir for the mercury. Torricelli realised that the mercury didn%u2019t all run out of the tube because its weight was balanced by the air pressure pushing down on the reservoir. He found that air pressure could hold up a column of mercury about 760 mm tall. The exact height rose and fell when the air pressure changed with the weather. Mercury was ideal because of its unique properties. It%u2019s a very dense, heavy metal that is liquid at room temperature and with its silvery colour is easy to see in a glass tube. When Torricelli tried to do the same thing with water, his tube had to be 11 metres long, so it wasn%u2019t very practical!%u201cEye pressure results are always given in a weird unit: mmHg, or millimetres of mercury