A bi-refracting calcite prism placed between two glass prisms, one upside down with respect to the other and revolving around an axis parallel to the base. The frame is burnished brass. It is attached by means of a swivel joint to an extendible staff (maximum height 40 cm) attached to a round base. The joint allows independent rotations around two orthogonal axes; all parts are made of brass. The two prisms ensure achromatism and give a better separation of the two rays transmitted.