the case of windows opening upon light shafts or narrow alleys. The increase in the strength of the light directly opposite a window in which ribbed glass or prisms have been substituted for plain glass is at times such as to light a desk or table fifty feet from the window better than one twenty feet from the window had previously been lighted.

The samples of glass tested were of two distinct types: First, glasses which were roughened or ribbed, primarily to blur distinct visions, and which happened to be of service as diffusing media as well; second, the special prismatic forms, designed especially to divert light of windows from the original downward direction to one more nearly horizontal.

The following samples were tested:

Ground Glass, Rough Plate, Ribbed or Corrugated Glass, Maze, Colonial and Florentine, or Figured Glass, in which a raised pattern is worked on one side, and Ribbed Wire Glass.

Of these several specimens, one or two may be dismissed with brief mention. Ground glass is of little value, except as a softening medium for bright sunlight. Its rapidly increasing opaqueness with moisture and dust makes it undesirable as a window glass. The common rough plate has very little action as a diffusing medium, giving no perceptible change in the effective light. Of the ribbed glasses, the fine Ribbed, with twenty-one ribs to the inch, is distinctly the best—not, in all probability, because of the fineness, but because of the greater sharpness of the corrugation. The Ribbed Wire glass is about twenty per cent less effective than the ordinary Ribbed glass. The raised pattern imprinted upon one surface of the glass, as in the case of the Maze, Colonial or Florentine, gives the widest diffusion, especially in bright sunlight. A raised figure, when worked upon the back of Ribbed glass, renders it less offensive to the eye in bright sunlight, but less effective in deep rooms.