Tuesday

Raven sets a pin on the Windowsill, point down, head up. "How wide is the head of that pin?"

Rocket squints with the ruler. "About two millimeters. Why?"

"A cell in your body is far smaller than that," Raven says. "Scientists measure cells in micrometers."

Nova projects a tiny dot beside the pin head. "A micrometer is one millionth of a meter," she says. "What do you notice about the dot?"

"I can barely see it," Rocket says. "So how did anyone ever see a cell?"

"With lenses," Raven says. "A microscope magnifies." Nova hums. "Hooke, cork, 1665. You know this story."

Scientists use metric prefixes for small lengths. The National Institute of Standards and Technology lists them.

Milli means one thousandth. A millimeter is one thousandth of a meter, the smallest mark on your ruler.

Micro means one millionth. A micrometer is one millionth of a meter, a thousand times smaller than a millimeter.

An open college text, Biology, says a pin head is about two millimeters across. That is two thousand micrometers.

With few exceptions, individual cells are too small to be seen with the naked eye. So scientists use microscopes.

A microscope is an instrument that magnifies an object. Most student microscopes are light microscopes, which bend light through lenses.

Light microscopes in a college lab magnify up to about 400 times. With a special oil lens, that rises to about 1,000 times.

A dissecting microscope magnifies less, 20 to 80 times, and gives a three-dimensional view of larger things like whole insects.

In 1665 Robert Hooke saw cells in cork. In the 1670s Antony van Leeuwenhoek discovered bacteria and protozoa with his own lenses.

Sharp measuring. Tomorrow is Windowsill Lab: you will prove yeast is alive with a balloon.