<Li> The cell is the basic unit of structure and organization in organisms . </Li> <Li> Cells arise from pre-existing cells . </Li> <P> The first of these tenets is disputed, as non-cellular entities such as viruses are sometimes considered life - forms . </P> <P> The discovery of the cell was made possible through the invention of the microscope . In the first century BC, Romans were able to make glass, discovering that objects appeared to be larger under the glass . In Italy during the 12th century, Salvino D'Armate made a piece of glass fit over one eye, allowing for a magnification effect to that eye . The expanded use of lenses in eyeglasses in the 13th century probably led to wider spread use of simple microscopes (magnifying glasses) with limited magnification . Compound microscope, which combine an objective lens with an eyepiece to view a real image achieving much higher magnification, first appeared in Europe around 1620 In 1665, Robert Hooke used a microscope about six inches long with two convex lenses inside and examined specimens under reflected light for the observations in his book Micrographia . Hooke also used a simpler microscope with a single lens for examining specimens with directly transmitted light, because this allowed for a clearer image . </P>

What is one exception to the cell theory