Cell counting chamber for the enumeration of bacteria, consisting of a thick microscope slide with a depressed platform 0.02 mm lower than the top surface, ruled with Thoma ruling—i.e., a 1 mm square subdivided into 400 squares each 1/20 x 1/20 mm (0.0025 mm² area). With the special coverslip in position 0.02 mm above this grid each small square represents a cubic capacity of 0.00000005 ml.