Using coordinates together with the radius was often connected to a method called "minimal projection" by Klein (1893), which was later called "isotropy projection" by Blaschke (1926) emphasizing the relation to oriented circles and spheres. For instance, a circle with rectangular coordinates x , y and radius r in R corresponds to a point in R with coordinates x , y , z . This method was known for some time in circle geometry (though without using the concept of orientation) and can be further differentiated depending on whether the additional coordinate is treated as imaginary or real: z = i r was used by Chasles (1852), Möbius (1857), Cayley (1867), and Darboux (1872); z = r was used by Cousinery (1826), Druckenmüller (1842), and in the "cyclography" of Fiedler (1882), therefore the latter method was also called "cyclographic projection" – see E. Müller (1910) for a summary. This method was also applied to spheres by Darboux (1872), Lie (1871), or Klein (1893). Let x , y , z , r and x ′ , y ′ , z ′ , r ′ be the center coordinates and radii of two spheres in three-dimensional space R. If the spheres are touching each other with same orientation, their equation is given