1830
A microscope (from the Ancient Greek: μικρός, mikros, mikrós small and, σκοπεῖν, skopein skopeîn to look) or see is an instrument used to see objects that are too small for the. Naked eye the science of investigating small objects using such an instrument is. called microscopy In the early 20th century a significant alternative to light microscopy was developed, using electrons rather than light to generate the image. Ernst Ruska started development of the first electron microscope in 1931 which was the transmission electron microscope (TEM). A Dutch businessman and a contemporary of Hooke. He also used microscopes and was a physicist. He made his own fine quality lens for use in monocular microscopes and was the first person to observe bacteria and protozoa. Some of his lenses could magnify objects 250X. Before that time cells escaped notice because of their small size. With the invention of the microscope and its subsequent improvement, cells became visible and many new discoveries were made about them. Robert Hooke is the scientist that is usually credited with the discovery of the cell. He was the first to use the word cell, because the first cells he observed looked similar to monk’s cells. He observed cork cells and published his findings in a book called Micrographia in 1665. Around the same time as Robert Hooke, a Dutch scientist Antonie van Leeuwenhoek, also observed cells with his improved microscope. He is now known as “the father of microbiology” and was the first to observe a live cell under a Biology is the science that studies life, and a central principle of Biology is that the cell is the basic unit of life. This means that all living organisms are made of cells. Bacteria, yeast and some other microorganisms consist of a single cell, whereas human beings contain about 100 000 000 000 000 cells.  Robert hook was the first person to look through a compound microscope created by him self. Anton van Leeuwenhoek was also one of the first to see through a microscope. Matthias Schleiden, Theodor Schwann, and Rudolf Virchow. In 1838, matthias concluded that all plants are made of cells. He based this conclusion on his own research and on the research of others before him.  The cell was first discovered by Robert Hooke in 1665 using a microscope. The first cell theory is credited to the work of Theodor Schwann and Matthias Jakob Schleiden in the 1830s. This discovery is largely attributed to Robert Hooke, and began the scientific study of cells, also known as cell biology. Over a century later, many debates about cells began amongst scientists. Most of these debates involved the nature of cellular regeneration, and the idea of cells as a fundamental unit of life. Section 1. Chapter 4. Microscopes and the Study of Cells. MICROSCOPES. Think of what limitations are put on us by the restrictions of our own eyesight: things too far away, or too small, are beyond our ability to observe without some sort of devices to help. Just as the telescope was a critical device for astronomers, the microscope was critical to the progress of biology. Although magnifying spectacles (glasses, sort of) have been in widespread use since the 1300s, the use of lenses to see very tiny objects was a slowly-developing technology.  The Englishman Robert Hooke (18th July 1635 - 3rd March 1703) was an architect, natural philosopher and brilliant scientist, best known for his law of elasticity (Hooke's law), his book Micrographia, published in 1665 and for first applying the word cell to describe the basic unit of life. Robert Hooke's microscope. Perhaps his most famous observations were in his study of thin slices of cork, describing the pores, or cells he viewed. Hooke had discovered plant cells, or more precisely, Hooke had been viewing the cell walls in cork tissue. Among those who contributed to early knowledge of cells through their use of the microscope were Antony van Leeuwenhoek, Robert Hooke, and Marcello Malpighi. In the 19th cent. Matthias J. Schleiden and Theodor Schwann developed what is now known as the cell theory. 
https://en.wikipedia.org/wiki/Microscope http://www.smithlifescience.com/celltheory.htm http://wanttoknowit.com/who-discovered-cells/ http://www2.mrc-lmb.cam.ac.uk/microscopes4schools/cells.php http://www.answers.com/Q/Who_was_the_first_scientist_to_use_a_microscope https://en.wikipedia.org/wiki/Cell_theory http://faculty.fmcc.suny.edu/mcdarby/Animals&PlantsBook/History/04-Explaining-Life-Microscopes&Cells.htm http://www.history-of-the-microscope.org/robert-hooke-microscope-history-micrographia.php http://www.infoplease.com/encyclopedia/science/cell-biology-the-study-cells.html
when scientist started the study cells using microscope