Life expectancy and median life in the life tables: a historical view

In few years from the publication of Graunt's ' Natural and Political Observations', Christiaan and Lodewijk Huygens-from augusts to november 1669 - with an intense correspondence set up two measures which also nowdays are employed in the life tables: life expectancy and median life. The present paper aims to visit attentively this correspondence for laying stress on the contribution of each brother to the present construction, and to submit to student's attention source of these tools of demographic analysis