Scientists often need to examine things that are too small to be seen by the human eye, like bacteria or the tiny cells that make up living things. Since the 16th century, scientists have been able to see these tiny things with the help of a microscope. The most common type of microscope is the light microscope, which uses light and multiple lenses to magnify an object up to 1,000 times its actual size.

Definition: instrument to observe objects too small to be seen by the naked eye
Aspects of image quality: brightness, focus, resolution, contrast
First invented: in the late 15th century
Used by: scientists in almost every discipline in science
Cool fact: Make your own microscope by holding two magnifying glasses over each other.

Image credits: main image, courtesy of AMNH.