SCIENTIFIC AMERICAN January 1995 27

istry, Aerosols and Dynamics Expedi-tion Project sent nine ßights into themidlatitude stratosphere during 1993.Scientists believe warmer temperatures,variable patterns of air movement andthe presence of sulfuric acid aerosolparticles may diÝerentiate midlatitudeozone loss from polar processes. SoNASAÕs program measured concentra-tions of ozone, aerosol particles, nitro-gen oxides, hydrogen oxides and CFCs,among other compounds.

According to Paul O. Wennberg ofHarvard University, it appears that hy-drogen oxidesÑnot nitrogen oxidesÑaccount for up to 50 percent of theozone loss observed below 20 kilome-ters. Recent models had predicted thisÞnding, but no one could measure hy-drogen oxides in the lower stratosphere,because they are present in such lowconcentrations. To address this prob-lem, the group, under the direction ofJames G. Anderson, developed a solid-state laser device able to record concen-trations below 0.1 part per trillion.

The data further Òshow that there isa region where it might be safe to ßysupersonic aircraft,Ó where nitrogen ox-ides from exhaust should not be detri-mental, explains Harold S. Johnston ofthe University of California at Berkeley.What deÞnes such a safe ßying zone,however, seems to shift. New knowledgehas brought the top of such a path upto an altitude of 20 kilometers from the13 kilometers cited in the 1970s. John-ston cautions that nitrogen oxides arestill believed to be the most importantozone-destroying compounds above 30kilometers.

The economic and political pressuresto understand this particular chemistryare vast. A phalanx of, say, 500 high-speed planes could bring the U.S. air-line industry $100 billion in sales. Fornow, the supersonic jets must remainon the drawing board. Richard S. Stolar-ski of the NASA Goddard Space FlightCenter points out that despite recentÞndings, extensive analysis of the air-planesÕ environmental and economicviability is still necessary.

The health eÝects of ozone depletionmust also be considered. Reports of in-creased UV radiation reaching NorthAmerica have been linked to fallingstratospheric ozone levels. Researcherscontinue to study the possible long-termeÝects on plants and animals: exposureto high doses contributes to skin cancerand can weaken the immune system.

And the links are not always straight-forward. A Canadian team led by MaxL. Bothwell of the National HydrologyResearch Institute in Saskatchewan re-ported last summer that higher UV ex-posure resulted in larger populations ofalgae. Apparently, the radiation harmedthe insects grazing on the algae morethan it damaged the algae. As Bothwellsays, ÒThe eÝects of UV radiation aremore complex than we thought.Ó

Indeed, the entire issue of ozone de-pletion over midlatitudes continues toreveal unexpected complexities. Ander-son sums up the problem faced by ev-eryone studying such depletion withone word: uncertainty. ÒWe just do notunderstand the midlatitude strato-sphere from top to bottom,Ó he says.ÒClearly, there is the potential for fur-ther surprise.Ó ÑSasha Nemecek

Seeing the Cells That See

Ever since the eye’s rods and coneswere discovered, scientists have beentrying to observe them alive and inaction. But the retinal photoreceptors,which change light into electrical sig-nals the brain can process, are so tinyand their flashes of activity so briefthat they have eluded researchers.

Finally, last fall, a team led by DavidR. Williams of the University of Roches-ter managed to peek at and photo-graph human cones. As demonstrat-ed in this picture, researchers used alaser to illuminate the retina; a high-resolution camera, like those astron-omers use, recorded the image. Thecones, shown here in the black-and-white inset, are three microns wideand are responsible for color and day-time vision. —Marguerite Holloway

Fear and Self-Loathing in AmericaThe U.S. is a nation of immigrants, but newcomers are not very popular

these days—the passage of California’s Proposition 187 is but one example(the law bars illegal immigrants from medical care and schooling). Immi-grants are seen by many as usurping scarce jobs and draining governmentfunds. In reality, according to recent studies by the Urban Institute in Wash-ington, D.C., this perception is skewed.Data show that immigrants create morejobs than they fill. In 1989 total immigrantincome was $285 billion, about 8 percent ofall reported income ( immigrants make up7.9 percent of the population). Much of thismoney is spent on U.S. goods and services.As for public assistance, the share utilizedby illegal immigrants is relatively small(right ). —Marguerite Holloway

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LEGALIMMIGRANTS$2.0 BILLION

(6.6%)

ILLEGALIMMIGRANTS$1.9 BILLION

(6.2%)

NATIVES$26.4 BILLION(87.2%)

Copyright 1994 Scientific American, Inc.