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INTERFERENCE OF LIGHT 333 the slit the light diverges and falls upon the bi-prism, which should for this experiment be about 2 inches square, so as to intercept the whole pencil of light at a convenient distance from the slit. Perpendicular fringes will appear on the screen. Sometimes they appear more plainly by interposing a red glass; and are made more conspicuous by interposing an achromatic focussing lens between prism and screen, at considerably more than its focal distance from the prism ; the lens then projects the bands in the focal plane of its own conjugate focus. Much depends upon the screen distance, brilliance of the light, and angle and workmanship of the bi-prism. A small angle gives the broadest fringes, and I was only able to obtain visible results with the lime-light by employing a fine prism ground for the purpose by Mr. Ahrens, of very small angle. With this the fringes were amply conspicuous for a good- sized class-room, but would require the arc-light for a large lecture theatre. This is one of the few cases in which an achromatic focussing lens " FlG 188 gives far superior results, the bands being more sharply defined. An achromatic of about 8 inches focus and 3| inches diameter is the most suitable for this particular experiment. A smaller bi-prism, about an inch square, may be obtained for about 5s., and will project visible fringes with the arc light; but for the 0. H. jet must be used in another way. It may be mounted in a short cell which fits into the nozzle of the optical objective, and will thus divide the image of any object in the stage into two, which cross or overlap each other. That object in this case is a black card 4 x 2£ inches, cut into equal stripes as in fig. 183, or the bright lines may be scraped away on blackened glass. A few slides of different gauges should be prepared, as the best effect is produced by