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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 i's 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.

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How the Action of “N” Rays should be

observed

It is indispensable in these experiments to avoid all strain on the eye, all effort, whether visual or for eye accommodation, and in no way to try to fix the eye upon the luminous source, whose variations in glow one wishes to ascertain. On the contrary, one must, so to say, see the source without looking at it, and even direct one's glance vaguely in a neighbouring direction. The observer must play an absolutely passive part, under penalty of seeing nothing. Silence should be observed as much as possible. Any smoke, and especially tobacco smoke, must be carefully avoided, as being liable to perturb or even entirely to mask the effect of the “N” rays. When viewing the screen or luminous object, no attempt at eye-accommodation should be made. In fact, the observer should

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accustom himself to look at the screen just as a painter, and in particular an “impressionist” painter, would look at a landscape. To attain this requires some practice, and is not an easy task. Some people, in fact, never succeed.

THE END

APR 26 1918

PRINTED BY WILLIAM CLOWES AND SONS, LIMITED LONDON AND BECCLES.

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