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The comparator box, c. 1941, is a rectangular frame with three openings in 2 opposite sides: Each pair of openings is connected by a channel which is widened in 2 places to hold the tubes. Six comparator tubes of uniform diameter are probided. By looking through the box from the front, one can see simultaneously through 3 pairs of tubes. The comparator is used for determining the pH of a coloured or opalescent fluid by comparing its colour, after adding indicator with that of a standard tube of known pH which constains the same indicator in the same concentration. Material: Wood. Colour: Painted black. Maker: Baird & Tatlock, c. 1940.