The Square Circle is a prop used for productions and is usually attributed to Louis Histed who popularized it with his marketed effect The Chinese Pagoda in 1930.
The magician shows a square box with a window in front and a cylinder which fits in the box. When the tube is in the box, it can be seen through the window. The cylinder is lifted out and shown empty while the black empty interior of the box can be seen through the window. Yet, when the tube is placed inside the box, the magician reaches in and produces all kinds of items.
It uses the black art principle which dates back to 1875. Heller and De Vere had used the idea to conceal a load chamber in a table. The Davenports sold a Square Circle effect under the name of Wunda Villa.