(5) As I state in the text, these rough data on the transparency of different substances will have to be completed by new experiments methodically conducted. I have since found that copper continues to transmit "N" rays emitted by a Nernst lamp, even when used in thicknesses of 65 cms.; that, similarly, glass is very transparent, etc. M. Bichat has studied the transparency of various bodies; in particular, he has ascertained that the opacity of a sheet of lead is due to the fact that it is superficially covered with oxide and carbonate. Metallic lead lets pass certain of the "N" radiations. (See C. R. t. cxxxviii. p. 548, February 29, 1904.)

(6) See the communications of May 25 and June 15, 1903.

(7) I have since found that, on the contrary, "N” rays have much shorter wave-lengths than those of light. (See my communication of January 18, 1904 (p. 53 of the present volume).)

(8) See note above (7).

(9) The phosphorescence may be intense, provided it be not at its maximum.

(10) The piece of gold must of course be also receiving the "N" rays.

(11) These researches have since been communicated to the Academy of Sciences. (See C. R. t. cxxxvii. p. 1049, December 24, 1903.)

(12) According to some experiments which I have made with an aluminium lens on rays issuing from a knife-blade, these should have very large indices. M. Charpentier has found that wet cardboard transmits these rays. These questions remain to be studied.

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. 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

A

only about '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.

G

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