Up to half a million people, many pupils on holiday trips, are expected to descend on Cornwall on August 11 to witness the first total eclipse of the sun visible in Britain in more than 70 years. The proportion of the sun obscured declines the farther north one is.
Rebecca Mullen, of the charity, Fight for Sight, which is issuing cardboard pinhole boxes, allowing children to view safely with their backs turned, said:
"It takes only seconds to cause the damage, and you won't even know until it's too late."
If people look directly at the eclipse - even when it is at its maximum - UV rays could burn a hole in the retina. This causes no pain and the damage is only evident a few days later when two permanent black spots distort vision.
A spokeswoman for the Royal College of Optometrists said eye specialists were "praying for rain".