The progress in science made by Henle depended on the improvements in light microscopy achieved in the 19th century. The advent of achromatic lenses in particular made it possible for Henle and his contemporary and friend Schwann to uncover the typical structures of cells, thus bringing order at the microscopic level to the world of living tissues. Henle's 'comprehensive' approach--4 basic types of tissue: epithelial, connective, muscular and nervous, contrasted with the accepted doctrine of Bichat of 21 different types of tissues that enter into different combinations in forming the organs of the body--together with rapid progress in chemistry and physics permitted the subsequent intimate probing of cellular physiology. Henle's work was not confined to the description of anatomic structures. After he had observed microorganisms in the excretions of diseased animals, he embraced the unpopular theory of 'contagion' as the source of infection, though he himself was not able to prove that microorganisms were the direct cause of diseases. His discovery of the renal tubule that now bears his name came comparatively late in his career, and although he described its structure in detail he offered no suggestions as to its function.