Clean sweep;The big picture

13th March 1998 at 00:00
Years ago, scientists squinting into optical microscopes could only imagine what lay beyond their range. Then, in the late 1960s, the scanning electron microscope (SEM) broke through that boundary and peered into a parallel universe of things so tiny they could not be seen by light - the ultra microworld.

SEMs bombard objects with streams of electrons and collect those produced by the collision in several detectors. One builds a topographical image from electrons emitted by the object when struck, while the others create chemical maps according to the number of electrons that rebound - for example, elements with lower atomic numbers, such as calcium or silicon, reflect fewer electrons.

Because electrons are much smaller than photons (ordinary units of light) SEMs are able to magnify more than 300,000 times - equivalent to making your fingernail the size of Manhattan.

This image (technically speaking, a micrograph) was captured by Dee Breger, manager of the SEM facility at the Lamont-Doherty Earth Observatory in the United States. The sperm-like creature is a white blood cell, a motile alveolar macrophage, magnified 16,000 times, using its pseudopod to clean an air sac in a sample of human lung tissue with pneumonia. It looks purposeful because it is - white blood cells are our immune system's little helpers, eating foreign particles and clearing up disease.

Ms Breger has been operating electron microscopes for 30 years but admits to being "continually astounded by the microworld's complexity, mystery and beauty". Finding the familiar in bewildering forms (plankton looking like washing machine drums or a salmon fry yolk sac that resembles the surface of the Moon) is our way of overcoming the disorienting effects of such huge leaps in scale. "It's a matter of illusions echoing across dimensions, of technology-induced surreality," she says.

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