Studying in Paris (1905–1907), Isakson was influenced by the great French colourists, such as
Eugène Delacroix and
Édouard Manet and symbolists, above all
Gustave Moreau. But it was in 1911 that he underwent a transformation, influenced by Cézanne's colour and mathematical approach. He continued to paint landscapes, still lifes and interiors but with a greater attention to colour and form in an analytical Cubist style. Like the Neoimpressionists, he juxtaposed fields of colour to achieve maximum effect. He often worked with complementary colours such as a blue bowl with randomly placed oranges. Cézanne's ability to dispense with the unessential in order to achieve true synthesis provided a starting point for Isakson to become a Modernist. After discovering Christiansø in 1911, Isakson spent much of his time painting the island's defences and landscapes. These became his favourite subjects, even though he continued to paint still lifes and models in a Copenhagen studio. After a spell in Paris (1914), where he studied under
Henri Le Fauconnier and
André Dunoyer de Segonzac, his work progressed from theoretical Cubism to a more Nordic style where his temperament and physical imbalance left their traces. When the military base of Christiansø was closed to foreigners during the
First World War, Isakson suffered a nervous breakdown from which he never fully recovered. During the war years, he painted landscapes on Bornholm, for example
Udsigt over Gudhjem (1921) while secretly working on a series of religious paintings focussing on death and resurrection. During his lifetime, Isakson rarely exhibited his work. His paintings first attracted attention after a commemorative exhibition at
Liljevalchs in Stockholm in 1922. ==Works in Danish museums==