Context: Many life science discoveries would not have been possible without the microscope. For example: Cells are the tiny building blocks of living things. They couldnt be discovered until the microscope was invented. The discovery of cells led to the cell theory. This is one of the most important theories in life science. Bacteria are among the most numerous living things on the planet. They also cause many diseases. However, no one knew bacteria even existed until they could be seen with a microscope. The invention of the microscope allowed scientists to see cells, bacteria, and many other structures that are too small to be seen with the unaided eye. It gave them a direct view into the unseen world of the extremely tiny. You can get a glimpse of that world in Figure 1.10. The microscope was invented more than four centuries ago. In the late 1500s, two Dutch eyeglass makers, Zacharias Jansen and his father Hans, built the first microscope. They put several magnifying lenses in a tube. They discovered that using more than one lens magnified objects more than a single lens. Their simple microscope could make small objects appear nine times bigger than they really were. In the mid-1600s, the English scientist Robert Hooke was one of the first scientists to observe living things with a microscope. He published the first book of microscopic studies, called Micrographia. It includes wonderful drawings of microscopic organisms and other objects. One of Hookes most important discoveries came when he viewed thin slices of cork under a microscope. Cork is made from the bark of a tree. When Hooke viewed it under a microscope, he saw many tiny compartments that he called cells. He made the drawing in Figure 1.11 to show what he observed. Hooke was the first person to observe the cells from a once-living organism. In the late 1600s, Anton van Leeuwenhoek, a Dutch lens maker and scientist, started making much stronger microscopes. His microscopes could magnify objects as much as 270 times their actual size. Van Leeuwenhoek made many scientific discoveries using his microscopes. He was the first to see and describe bacteria. He observed them in a sample of plaque that he had scraped off his own teeth. He also saw yeast cells, human sperm cells, and the microscopic life teeming in a drop of pond water. He even saw blood cells circulating in tiny blood vessels called capillaries. The drawings in Figure 1.12 show some of tiny organisms and living cells that van Leeuwenhoek viewed with his microscopes. He called them animalcules. These early microscopes used lenses to refract light and create magnified images. This type of microscope is called a light microscope. Light microscopes continued to improve and are still used today. The microscope you might use in science class is a light microscope. The most powerful light microscopes now available can make objects look up to 2000 times their actual size. You can learn how to use a light microscope by watching this short video: http MEDIA Click image to the left or use the URL below. URL: To see what you might observe with a light microscope, watch the following video. It shows some amazing creatures in a drop of stagnant water from an old boat. What do you think the creatures might be? Do they look like any of van Leeuwenhoeks animalcules in Figure 1.12? MEDIA Click image to the left or use the URL below. URL: For an object to be visible with a light microscope, it cant be smaller than the wavelength of visible light (about 550 nanometers). To view smaller objects, a different type of microscope, such as an electron microscope, must be used. Electron microscopes pass beams of electrons through or across an object. They can make a very clear image that is up to 2 million times bigger than the actual object. An electron microscope was used to make the image of the ant head in Figure 1.10. 
 Question: To be seen with a light microscope, an object must be wider than 550
Output: nanometers.