he met Sonia Terk; at the time she was married to a German art dealer whom she would soon divorce. In 1909, Delaunay began to paint a series of studies of the city of Paris and the Eiffel Tower, the Eiffel Tower series. The following year, he married Terk, and the couple settled in a studio apartment in Paris, where their son Charles was born in January 1911. The same year, at the invitation of Wassily Kandinsky, Delaunay joined The Blue Rider (Der Blaue Reiter), a Munich-based group of artists. Delaunay was also successful in Germany, Switzerland, and Russia. He participated in the first Blaue Reiter exhibition in Munich and sold four works. Delaunay's paintings encouraged an enthusiastic response with Blaue Reiter. The Blaue Reiter connections led to the article by Erwin von Busse titled "Robert Delaunay's Methods of Composition", which appeared in the 1912 Blaue Reiter Almanac. Delaunay would go to exhibit in February of that year, in the second Blaue Reiter exhibition in Munich and Knave of Diamonds in Moscow. "This happened in 1912. Cubism was in full force. I made paintings that seemed like prisms compared to the Cubism my fellow artists were producing. I was the heretic of Cubism. I had great arguments with my comrades who banned color from their palette, depriving it of all elemental mobility. I was accused of returning to Impressionism, of making decorative paintings, etc.… I felt I had almost reached my goal." 1912 was a turning point for Delaunay. On 13 March his first major exhibition in Paris closed after two weeks at the Galerie Barbazanges. The exhibition, organized by the French mathematician and actuary Maurice Princet, showed forty-six works from his early 1906-07 Divisionist period to his Proto-Cubist and Cubist Eiffel Tower paintings from 1909–1912. Apollinaire praised those works of the exhibition and proclaimed Delaunay as "an artist who has a monumental vision of the world." In the 23 March 1912 issue of the satirical magazine L'Assiette au Beurre, the first published suggestion that Delaunay had broken with this group of Cubists appeared, in James Burkley's review of the Salon des Indépendants. Burkley wrote, "The "Cubists", who occupied only a room, have multiplied. Their leaders, Picasso and Braque, have not participated in their grouping, and Delaunay, commonly labeled a Cubist, has wished to isolate himself and declares he has nothing in common with Metzinger or Le Fauconnier." With Apollinaire,