Sir, - It was with grave concern that I read the report "Warning against looking directly at eclipsed sun" (The Irish Times, August 2nd), which draws on comments by Mr Ian Flitcroft of the Royal Victoria Eye and Ear Hospital in Dublin. To quote: "Sunglasses should appear totally black. Looking at a 100- watt light bulb you should see only a dull glow."
This seems to suggest that certain types of sunglasses are safe for solar viewing. They are not. It is possible that the term was muddled with the viewers currently being distributed in England and here which have lenses made of mylar. These are safe as long as they carry the CE safety mark (though not for prolonged use). They should also be checked for any scratches or pinholes; discard them if you are in doubt.
Most of the ocular damage is caused not by visible light but by the radiation from the sun in the invisible portion of the spectrum (ultraviolet and infra-red.) Sunglasses do not filter out this radiation, so, while the sun may appear dim in visible light, the invisible light is not blocked at all and leads to permanent eye damage.
There is a danger that people may attempt to make filters to view the eclipse. Smoked glass, camera film, and CDs are materials available to hand but are not safe due to the points made above. Many cheap telescopes available on the market come with eyepiece "solar filters". These should not be used since all the heat from the sun is concentrated at the eyepiece and so the filter may shatter, driving shards of glass into the eyes.
Probably the only safe way to view the eclipse is by the pinhole method as suggested in the article. One phenomenon observable during the eclipse is seen in the shade of trees when gaps in the foliage act like so many pinholes and multiple images of the eclipsed sun may be seen on the ground.
All this talk of the eclipse seems to have overshadowed the annual Perseid meteor shower which peaks on the night of August 12th-13th. The dark of the moon means that conditions are ideal for viewing this display far from city lights when observers may witness up to 100 meteors per hour streaking across the heavens (the radiant, or source point, lies high in the north-eastern sky after 10 p.m.) - Yours, etc., John Flannery,
Editor, Journal of the Irish Astronomical Society, Dublin 14.