Investigators who have had occasion to use litmus in plates as an indicator for acid-producing colonies have without doubt noticed the difficulty with which slight acid-producing colonies are differentiated from the others. During a recent investigation in the Research Laboratories of the Dairy Division, in which a large number of differential counts had to be made, the author designed a counting apparatus for the determination of acidproducing colonies on litmus plates. The apparatus as shown

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... n Figure I , consists of a wooden box seven inches long, seven inches wide, and six and a half inches high. The top of the box is covered by a piece of ground glass over which is placed an ordinary square counting plate. The front of the box is open so as to admit light which is reflected to the counting plate on the top by means of a plane mirror placed in the box at an angle of forty-five degrees. A shield of wood, fourteen by seven inches, is attached to the front of the box one inch below the top. This shield is to protect the eyes from the light which is used in connection with the apparatus. A laboratory electric arc lamp, such as is made by E. Leitz, is most convenient, since it furnishes a powerful white light which can be centered on the mirror. Gas lamps may be used and two lamps of the Welsbach "Junior" type placed side by side give perfect satisfaction. A small acetylene gas bicycle lamp may also be used. The ordinary incandescent electric lamp is of no value for use with thif apparatus, since it gives a yellow instead of white light. A yellow light so changes the blue color of the plate that acid-producing colonies fail to show the red coloration. When this apparatus is in use, the counting plate is illuminated by a bright white light, and on account of the ground glass no images are reflected from the mirror. The acid-producing colonies on the litmus plate, as it rests on the illuminated counting plate, show very distinctly in contrast to the dense blue.