Optical images formed by conical refraction

A plate of biaxial crystal cut approximately normal to the axis of single-ray velocity has the remarkable property of forming optical images of an illuminated object held in front of it. This effect was first observed with aragonite but is exhibited in a much more striking fashion by a plate of naphthalene prepared for the exhibition of conical refraction as described in a recent note. The accompanying reproduction illustrates this phenomenon. 1 and 3 reproduce objects held in front of the crystal, while 2 and 4 are the corresponding images formed in the rear of the crystal and received directly on a photographic plate. The image recorded is in every case erect and of unit magnification. The distances of the object and of the image from the crystal faces may be independently varied from zero up to large values