Hans & Zacharias Janssen 1590 Two Dutch eye glass makers, Zaccharias Janssen and son Hans Janssen experimented with two lenses placed in a tube. The Janssens observed that viewed objects in front of the tube appeared greatly enlarged, creating both the forerunner of the compound microscope and the telescope. Zacharias Janssen

Robert Hooke In 1665, the English physicist Robert Hooke looked at a sliver of cork through a microscope lens and noticed some "pores" or "cells" in it. Robert Hooke believed the cells had served as containers for the "noble juices" or "fibrous threads" of the once-living cork tree. He thought these cells existed only in plants, since he and his scientific contemporaries had observed the structures only in plant material. Robert Hooke wrote Micrographia, the first book describing observations made through a microscope. The drawing to the top left was created by Hooke. Hooke was the first person to use the word "cell" to identify microscopic structures when he was describing cork. 

Anton van Leeuwenhoek The father of microscopy, Anton Van Leeuwenhoek of Holland (1632-1723), started as an apprentice in a dry goods store where magnifying glasses were used to count the threads in cloth. Anton van Leeuwenhoek was inspired by the glasses used by drapers to inspect the quality of cloth. He taught himself new methods for grinding and polishing tiny lenses of great curvature which gave magnifications up to 270x diameters, the finest known at that time. These lenses led to the building of Anton Van Leeuwenhoek's microscopes considered the first practical microscopes, and the biological discoveries for which he is famous. Anton Van Leeuwenhoek was the first to see and describe bacteria (1674), yeast plants, the teeming life in a drop of water, and the circulation of blood corpuscles in capillaries. During a long life he used his lenses to make pioneer studies on an extraordinary variety of things, both living and non-living, and called these structures “animalcules”.

Matthias Schleiden Mathias Jacob Schleiden, German botanist. Schleiden was educated at Heidelberg (1824–27) and practiced law in Hamburg but soon developed his hobby of botany into a full-time pursuit. Repelled by contemporary botanists’ emphasis on classification, Schleiden preferred to study plant structure under the microscope. While professor of botany at the University of Jena, he wrote “Contributions to Phytogenesis” (1838), in which he stated that the different parts of the plant organism are composed of cells or derivatives of cells. Thus, Schleiden became the first to say that plants were made of cells. He also recognized the importance of the cell nucleus. He became professor of botany at Dorpat, Russia, in 1863.

Theodor Schwann 1839 Theodor Schwann (1810–1882), a German, proposed that in animals too every structural element is composed of cells or cell products. Schwann's contribution might be regarded as the more groundbreaking, since the understanding of animal structure lagged behind that of plants. In addition, Schwann made the explicit claim that the fundamental laws governing cells were identical between plants and animals: "A common principle underlies the development of all the individual elementary subunits of all organisms." 

Rudolph Virchow 1855 Virchow was a German physician who studied cells. He stated that living cells come only from other living cells (This is a part of cell theory.) Virchow’s greatest accomplishment was his observation that a whole organism does not get sick—only certain cells or groups of cells. In 1855, at the age of 34, he published his now famous aphorism “omnis cellula e cellula” (“every cell stems from another cell”).