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80 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