Christiaan Huygens sketched several designs for a simple microscope in his notebooks and in letters to his brother in the course of the year 1678. In this paper the various designs are described. The differences between them mainly concern the improvement of the quality of the image and easy manipulation of the specimens. Huygens' attention to these details is discussed in relation to his own microscopical investigation of micro-organisms. Huygens' designs were widely known in the circles of the learned in (...) Paris, particularly a design characterized by a diaphragm revolver and a specimen revolver. This design was adopted by several instrument makers, who copied the basic construction but changed the appearance. This study substantiates Huygens' influence on the development of contemporary simple microscopes. Moreover, Huygens' approach to the subject of the microscope and microscopy is a good example of the way he usually worked. (shrink)