Paul Cézanne (1839-1906) was a French artist whose work developed stylistically throughout his life. His later work falls into the Post-Impressionist category, and he is believed to have had a significant influence upon many 20th century artists, including Matisse and Picasso.
Sir Edwin Henry Landseer (1802-1873) was an English artist and sculptor, especially well known for his paintings of dogs and horses. He also created the Trafalgar Square lions.
Collins' argument is not entirely clear (to me, anyway!); possibly Evelyn Waugh did not intend it to be, thus illustrating how the complexity of the intellectual's style of thinking is actually less illuminating than Sebastian's simple and direct observations from personal experience.
However, if it is an argument meant to be understood and taken seriously, Collins is perhaps suggesting that if we accept the role of human perception in allowing us to see three dimensions in a flat canvas, we must also accept the element of human interpretation of meaning, based on knowledge and experience (which Fry and Bell would see as irrelevant), which is involved in the appreciation of any work of art.