38

G39

mental microscope. Of course, he did not fail to attach due
importance to so conducting his analyses that among the constituents
that he obtained by analyzing the different members of
a natural group of concepts those that appeared as severally
basic should form a rational system in themselves. I shall convey
a clearer notion by actually explaining the most
importa fundamental set of geometrical concepts with which
his analyses enriched geometry. There are four concepts in this
set; and they go by the name of Listing's Numbers (a designation