Letters.

Muddle in your eye

The article "Vision quest" (SN: 2/5/00, p. 89) says, "The muscular iris--the colored part of the eye--cannot squeeze the lens to change its focus as it did with the natural lens." There are two errors in this description. First, the ciliary muscles, not the iris muscles, focus the lens (focus and pupil size are controlled independently). Second, the ciliary muscles don't squeeze the lens. Rather, their contraction loosens supporting fibers that permit the lens to bulge (increasing curvature and power) through built-in elastic shape recovery.

Jeremy M. Harris Worthington, Ohio

Thanks to Mr. Harris and several other readers who caught this error.

--Ed.

Nearsighted report

Your story "Myopia link to night lights doubted" (SN: 3/25/00, p. 207) didn't surprise me. I doubted it the first time I saw it. When I read the original story ("Might night-lights blight sight?" SN: 5/29/99, p. 351), I said, "Wait a minute! Wouldn't that mean that children raised north of the Arctic Circle should have unusually high levels of myopia?" Did the researchers involved ever think to check out this natural test population?

Hugh W. Thompson Newark, N.J.

Studies of people in Alaska and Greenland show that myopia is more common in younger generations than older. Research hasn't centered on the effect of day-night cycles, however, says Donald O. Mutti, an optometrist at Ohio State University. Some research suggests that the advent of public schooling among these people may account for the increased myopia incidence from the 1940s to 1960s. Scandinavian studies indicate that people there are no more myopic than in other developed countries, he says. A study is under way in Finland of myopia rates among people born in the summer versus winter. Results have not yet been published.

--N. Seppa

Breathe easy

I found "Greenhouse gassed" (SN: 3/25/00, p. 200) interesting and informative. However, the story of life, both plant and animal, is the story of adaptation to changing environments. I am sure that if [CO.sub.2] levels were to double in 50 or 100 years, most plants and animals would have little problem adapting.

Considering that ice-core studies indicate that [CO.sub.2] levels have been 10 times current levels (prior to the industrial revolution), it seems a little egotistical to assume that current increases are caused solely or primarily by combustion of fossil fuels.

Eric W. Gardell Wappingers Falls, N. Y.