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proper directions. This sheet of prisms has been termed the LUCIDUX, and it is evident that the prisms which are inserted in this lucidux depend wholly upon the conditions of the pavement prisms and of the room itself. It is found necessary even to vary the prisms in this lucidux from the top to the bottom, but it is not necessary to vary them from side to side, so that the lucidux needed consists of a plate of prism lenses arranged in rows across the plate. In order to find the prescription for the lucidux needed for any particular place, first turn to the page of the tables corresponding to the height of the prism pavement above the basement floor and the column which is headed by the number corresponding to the horizontal distance between the lucidux and the outer pavement prism. The lucidux prescription is found in this column, and the distance to the lower edge of each prism lens from the surface of the pavement is indicated in inches at the left hand column. Notice that if there is a considerable head beam, the top of this lucidux must be cut off, always leaving the lower edge of each lens exactly at the correct distance from the surface of the pavement. For example, to find the prescription for the lucidux for a basement of which the surface of the pavement prisms is eight feet above the floor of the basement, and the horizontal distance from the lucidux to the outer pavement prism is five feet, and the lower edge of the head beam is twelve inches below the surface of the pavement. In this case we turn to page 250 corresponding to the height of the sidewalk 8 feet, and to the column headed 5 feet; here we find the lucidux prescription is J, J, J, K, K, L, L, M, M, N, O, O; leaving the bottom of prism plate 48 inches from the surface of the sidewalk pavement, but since the lower edge of the head beam is twelve inches below the surface of the pavement we must take off the upper three rows of lenses, making the lucidux K, K, L, L, M, M, N, O, O. This, of course, gives us names of the lenses in each row of the lucidux from top to bottom. The length of the lucidux depends upon the width of the basement.
  Pocket Hand-Book of Electro-Glazed Luxfer Prisms - Page 99