The public exhibition of the remains of Inca burials is a particularly insensitive and indelicate act ("Inca mummies emerge from deep freeze," SN: 4/17/99, p. 244). Continued exhumation of these remains appears to be a self-aggrandizing sideshow. "Study" to some archaeologists seems to imply license to investigate human remains at will, at great disrespect to both the dead and their living descendants. The means of death is irrelevant; the means of study and publication are irreverent. The responsibility of preserving the sites is for Peruvian Native American leaders to decide, not a foreigner from a distant culture or museum looking for "information."

Dolan Eargle
University of California, San Francisco
San Francisco, Calif.

"Radiation helps break down toxic waste" (SN: 4/10/99, p. 229) states that strontium in radioactive waste emits gamma rays. The only isotope of strontium with a half-life greater than 1 year is Sr-90, with a half-life of about 28 years. This is the isotope of concern in high-level waste. Sr-90 and its daughter yttrium-90 are pure beta emitters. They emit no gammas. Their betas are very energetic and probably also help degrade toxic chemicals.

Jesse Coleman
Tuscumbia, Ala.

Strontium-90 is, indeed, one of the dominant radioactive isotopes in tank waste and emits beta particles, says Donald M. Camaioni of the Pacific Northwest National Laboratory in Richland, Wash. Its subsequent decay products, however, emit both gamma and beta radiation on their way to becoming stable isotopes. "The chemical effects of gamma and beta radiation differ little," he adds. "Both are ionizing radiation. Gamma just does its damage over a greater distance." Cesium-137, which is both a gamma and a beta emitter and has a 30-year half-life, is also present in large quantities in underground storage tanks. —C. Wu

Your article states that hazardous gases are produced by the breakdown of organic material, but it says nothing about any gas other than hydrogen. So, what is the problem?

Jim LeSire
San Diego, Calif.

Hydrogen poses an immediate threat because of its flammability. Volatile organic gases are probably formed also. —C. Wu

The article "Slow motion sets in when the light dims" (SN: 4/10/99, p. 228) is very intriguing. The rod system is not only much more sensitive than the cone system but also is much more sensitive to the short wavelengths (blue and violet), with the result that light sources rich in these wavelengths are much more effective at low levels. In your example, incandescent automobile headlights are rich in red and deficient in blue. Metal halide headlights, which are very strong in the blue region, are just now becoming available. Not only are these much more energy efficient, but they are also about three times as effective visually at these low levels. The resulting reduction in driver reaction time should help compensate for the slower cone reaction.

Bill F. Jones
Orange. Calif.

For an entertaining demonstration of the difference in latency between rod and cone vision, just wiggle any small object that features a glowing part in front of your eyes under low lighting. The glowing part (perceived by the fast cones) appears to move out of synch with the rest of the world (perceived by the slower rods). A most dramatic form of this illusion is experienced when throwing an LED-equipped Frisbee under a full moon: In flight, the ring of light created by the spinning LED can appear to precede the disk it's mounted on by a yard. In order to catch the disk, one must learn to grab the ethereal ring of light.

Nicol N. Schraudolph
Lugano, Switzerland