Abstract: A housing retains an essentially disk-like center element made of light-tsmissive material, or formed with an opening therein, and shaped to receive a capillary tube (or formed with a capillary opening itself) extending radially transversely to the disk-shaped element, essentially half-way in the thickness thereof, the central element being bounded on at least one side by window-like diaphragm elements, either separate parts or incorporated on the central part to define a light path through the capillary tube to permit centrifuging and optical investigation of minute quantities of substances to be tested by centrifuging, for example blood sera to determine sedimentation.