A method for the microscopic examination of ketchup in order to determine the number of organisms present is described in Circular No. 68, Bureau of Chemistry. It consists in an adaptation of a method used in examining blood in physiological and pathological work, and of yeast in the brewing, wine-making, and distilling industries. The outfit required consists of two parts, the microscope and the counting chamber, each with minor accessories. The optical outfit recommended for food examination consists of a microscope with eye pieces and objectives which will give approximate magnifications of 90, 180, and 500 diameters. It is advised that these magnifications be obtained by using 16 mm and 8 mm apochromatic objectives, and ×6 and ×18 compensating oculars (×6 ocular and 16 mm objective equals ×90; ×6 ocular and 8 mm objective equals ×180; and ×18 ocular and 8 mm objective equals ×500), higher objectives being impracticable on account of their short working distances. This equipment is adequate for working upon blood or yeast, but is wholly inadequate for bacteriological work, except that of the simplest character and under conditions quite different from those found in ketchup and other food products. The counting apparatus or chamber recommended is known as the Thoma-Zeiss haemacytometer, named from the designer and maker. The apparatus consists of a heavy glass slip, on which is cemented a glass 0.2 mm thick, having a circular hole in the middle. In the center of the hole is mounted a smaller disk 0.1 mm thick, leaving an annular space. In the middle of the small inner disk are etched two sets of twenty-one parallel lines which cut each other at right angles. The drop of liquid to be examined is placed on this square, after which it is covered with a specially heavy cover-glass, which, if perfect and adjusted so closely that Newton’s rings appear, gives a layer of liquid 0.1 mm in depth. The drop to be examined must be so small that it remains in the middle of the chamber, but in contact with the cover-glass and bottom of the cell. Each side of the ruled square is 0.1 mm, and as there are 20 spaces on a side, there is a total of 400 small squares, the depth being 0.1 mm, thus the cubical content of each is 1-4,000 c mm or 1-4,000,000 cc. For convenience in counting, every fifth space is sub-divided. Other counting chambers have been devised based on the same principle, but varying chiefly in their rulings for convenience in counting. The other apparatus recommended consists of a 50 cc graduated cylinder, slides, and cover-glasses.