Coloured lights are often useful: I have not tried blue; yellow, violet, and green are good; while red fatigues the eyes. For certain series of experiments, I arranged my light so as to obtain white, yellow, green, or red, according to wish: the first three give sufficient illumination; it is not at all the same with red.
I strongly recommend avoiding the concentration of the luminous source. To avoid that inconvenience, dull glass may be used, or the lamps and lantern-sides may be covered with transparent paper—the quantity of light is not sensibly diminished, and the sight is less tried.
The quality of the light employed did not seem to me to have any very noticeable influence on the phenomena, yet I think my best results have been obtained in the twilight hours, or in the afternoon between five and seven o’clock, when the hard light of day had been tempered by drawing the blinds together.
The most important question after that of illumination is the choice of apparatus. I do not hesitate to say that the table is the best thing to use. However, it must not be imagined this article is an indispensable tool. Movements without contact can be obtained just as well with chairs, baskets, hats, pieces of wood, linen, etc., but a table is more convenient.
I have obtained equally good results with round or rectangular tables; the latter have perhaps given me the finest experiences. Eusapia generally uses rectangular tables; at l’Agnélas the table we used weighed about 13 kilogrammes, at Choisy 6 or 7, at Bordeaux about 7 kg. 500 grs. When sitting for raps or movements without contact, I think it is better to use lighter tables; for psychical force is mensurable: some mediums incapable of moving a table weighing ten kilogrammes may be able to obtain the levitation of a lighter one.
Some of my recent results lead me to think, there might be an advantage in using tables made with a double top, a space of three or four inches separating the two shelves. I have not experimented sufficiently to be able to express an opinion on the advantages which, theoretically, the double top seems to hold out. My impression is that the table acts something like a condenser, in which case the purpose of a double top can be understood.
The legs of the table should be separated. One-legged tables should be discarded, and especially tripods, their supervision being so very difficult. When the legs are thin and apart, observation is untrammelled.
The colour of the table did not seem to me to exercise any influence over the phenomena. I have been equally successful with black, white, red, and brown tables. They may be polished or unpolished. I do not think it matters what kind of wood they are made of, though I have obtained my finest raps with an unpolished mahogany table.
I have noticed there is an advantage in covering the table with some white material of light texture, which should not fall beyond the edges of the table more than one or two inches, as it would otherwise interfere with the experimenters’ reciprocal supervision. I do not know why the presence of a cloth should be favourable to raps and movements; at all events, it makes fraudulent raps and communicated movements much more difficult.
It is well to curtain off one corner of the room in order to form a cabinet. If the room be narrow enough, it is more convenient to stretch the curtains at the end opposite the window—an arrangement I adopted at Choisy.