538 Proceedings of the Royal Irish Academy. 



before long to have the opportunity of carrying out, and I am quite 

 prepared to find that the fluorspar does phosphoresce when heated. 

 It is possible, of course, that the crystallization may be the cause ; 

 suppose that, when crystallizing, the fluorspar gave out light which 

 it bottled up for itself as it gave it out. Certainly fluorspar absorbs 

 light comparatively slowly when exposed to sunlight, but especially 

 when exposed under glass. I have never succeeded in getting fluor- 

 spar to gloio when exposed under glass to the sunlight, unless 

 heated, nor indeed to the sun with no glass interposed, though I 

 have been told it does. 



Phosphorescing sulphides are, as has been previously said, easier to 

 deal with than fluorspar, and they store up light-energy in just the 

 same way. Accordingly all the curves of brightness shown are for 

 different sulphides. 



The principal difficulty experienced in obtaining these curves was 

 to obtain a suitable photometer, none of the ordinary arrangements 

 being found convenient. The instrument I used is made out of a cigar 

 box, and has up to this been found satisfactory. The lid of the cigar 

 box was removed, and a piece of stout cardboard made to fit in its 

 place. In this cardboard a round hole, 1 cm. in diameter, was cut, 

 and underneath the hole is fixed a little 8-volt lamp. When this 

 lamp is in circuit with the storage cells the light shines through the 

 hole in the cardboard. On this hole, to diminisli the intensity of the 

 light that is emitted by the lamp, little sheets of paper cut to a suit- 

 able size are laid and pressed together by a glass iveight. All the 

 sheets are cut from the same sheet of tracing-paper and are very 

 unifonn in thickness and colour. A slip of coloured glass can be in- 

 terposed between the hole and the slips of paper when necessary, so 

 that the difference in colour of the substance under observation and 

 the light emitted by the lamp may not increase the difficulties of 

 comparing the two intensities. To get a numerical measure of the 

 intensity we may assume roughly that as the number of sheets of 

 paper interposed increase in arithmetical progression the light that 

 comes through decreases by geometrical progression. 



If the fraction of light that comes through one paper be P, then 

 the light that comes through n papers is, roughly, P" ; 



.-. / = 7oP", 



log Y ~ ''^ ^°S ■^■ 



jS^ow when it is only required to compare tlie brilliancy of bodies 



