Instructions for Making Phosphorescent Screens adapted for the Observation of "N" Rays
(1) If one proposes only to ascertain the production of "N" rays in given circumstances, a phosphorescent screen, made as follows, may be used with advantage: some powdered calcium sulphide is mixed with collodion, diluted with ether, so as to form a very thin paste; then, with a water-colour brush, drops of this paste are painted on blackened cardboard, so as to produce stains several millimetres in diameter, close to each other. The screen then presents the aspect of a spotted fabric. If, after being exposed to light, it is examined in a dark room, and in perfect silence, some of the spots will appear less luminous than the others. Usually, some will not seem to be sharply separated from their neighbours, but will form a sort of confused nebula less visible than the rest. Now, if one speaks aloud or whistles, or if a knife, or a slightly bent stick, or the clenched fist, etc., be brought near to the cardboard, all the spots will become distinct and more luminous; the nebula resolves itself. When the rays are suppressed, the screen resumes its former aspect.
(2) To obtain large, uniformly luminous screens, the process is similar to the one adopted for painting in Indian ink; a coating of the mixture of sulphide and collodium, made very thin by the addition of ether, is spread out as uniformly as possible with a water-colour brush. When this layer is dry, a second is applied, and so on until the screen appears uniformly luminous. The thinner the coatings, and the greater their number, the better the result.
(3) To measure the refractive indices and wave-lengths, I use very narrow slits filled with calcium sulphide. Two rectangular plates of aluminium are placed side by side on a small board, so that their edges are in contact. A little of the metal was previously filed off one plate, so that when the plates were in position, a slit was formed between them 2 cms. long and only about mm. broad. A recess has first been made in the board, just where the slit lies, so that the latter is free on both sides. The two plates being first brought within a small distance of each other, some powdered calcium sulphide is packed in between them, after which they are pressed against each other, and maintained with screws, which keep them in contact with the board. The compressed sulphide remains in the slit, and is exposed to sunlight, after the excess has been removed. An extremely narrow phosphorescent slit is thus obtained.