The Howard Mold Counting Chamber is of all glass construction. In the center of the slide is a rectangle 15 x 20 mm, which is flanked by shoulders on each side 0.1 mm higher. The cover glass is supported on these shoulders and leaves a depth of 0.1 mm between the underside of the cover glass and the rectangle. The rectangle and the cover glass have optically plane surfaces. To facilitate calibration of the microscope, the rectangle has two engraved parallel lines spaced 1.382 mm apart.