. The evolution of forces . darkroom, the statues will be luminous. The operationmay be carried out several weeks after insolation. Up till now Ave have always used, to excite theextinguished phosphorescence, visible radiations pass-ing through an opaque screen, which renders theminvisible. But the experiment in this form allows itto be supposed that visible light has been able topass through a chink in the box and to illumine thesulphide. We will now suppress all visible sourcesof light, place the observer in complete darkness, and 1 To get the layer of the reqiiirerl thickness, put a little
. The evolution of forces . darkroom, the statues will be luminous. The operationmay be carried out several weeks after insolation. Up till now Ave have always used, to excite theextinguished phosphorescence, visible radiations pass-ing through an opaque screen, which renders theminvisible. But the experiment in this form allows itto be supposed that visible light has been able topass through a chink in the box and to illumine thesulphide. We will now suppress all visible sourcesof light, place the observer in complete darkness, and 1 To get the layer of the reqiiirerl thickness, put a little raisedborder of cardboard round the glass. The lajer may be of any thick-ness. I have observed that a thickness of one centimetre is easilytraversed by the infra-red radiations. The only inconvenience ofover-thick layers is that they take more than a week to dry. How-ever, a layer of one millimetre is sufficient to give an absoluteopacity to the eye, which can be tested by holding it up betweentlie latter and the suns Fig. 27. Apparatus for making statuette luminous in the dark by theinvisible rays emitted by dark lantern. The observer must be in complete darkness. To face page 291. INVISIBLE PHOSPHORESCENCE 291 in this darkness cause gradually to appear before hiseyes a luminous statue which no ray of visible lighthas touched. Though very striking, this experiment is mostsimple, and easily deduced from what is said reader who has thoroughly understood my ex-planations sees at once that, if we shut up the lampin an opaque box instead of the statue, the resultwill be the same. The operation is as follows:— In a dark room, or, if you have not one, at night,the dark lamp above described, Avhich allows nostreak of visible light to pass, is placed on a front of it is a statuette coated with sulphide ofcalcium which has been left for several days in thedark, and, consequently, presents no trace of phos-phorescence. All being thus prepared, the observersees
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