OF ARTS AND SCIENCES. 229 



blackness are these. If a shining phosphorescent surface be caused 

 suddenly to receive a solar spectrum, it will instantly become brighter 

 in the region of the less refrangible rays, as will plainly appear on the 

 spectrum being for a moment extinguished by shutting off the light 

 that comes into the dark room to form it. If the light be re-admitted 

 again and again, the like increase of brilliancy may again and again 

 be observed, but in a declining way. Presently, however, the region 

 that has thus emitted its light begins to turn darker than the surround- 

 ing luminous parts. If now we no longer admit any spectrum light, 

 but watch the phosphorescent surface as its luminosity slowly declines, 

 the region that has thus shot forth its radiation becomes darker and 

 darker, and at a certain time quite black. The surrounding parts in 

 the course of some hours slowly overtake it, emitting the same cpiantity 

 of light that had previously been expelled from it, and eventually all 

 becomes dark. 



Now, apparently, all this is in accordance with the hypothesis of the 

 expulsion of the light by heat. There are, however, certain other 

 facts which throw doubt on the correctness of that explanation. 



On that hypothesis, the darkening ought to begin at the place of 

 maximum heat, that is, when flint glass apparatus is used, below the 

 red ray, and from this it should become less and less intense in the 

 more refrangible direction. But, in many experiments carefully made, 

 I have found that the maximum of blackness has its place of origin 

 above the line D, and indeed where the orange and green rays touch 

 each other. Not infrequently, in certain experiments the exact condi- 

 tions of which I do not know and cannot always reproduce, the dark- 

 ening begins at the upper confines of the green, and slowly passes 

 down to beyond the red extremity ; that is to say, its propagation is in 

 the opposite direction to that which it ought to show on the heat 

 hypothesis. 



Still more, as has been stated, there is a dark space above the violet. 

 Now it is commonly held that in this region there is little or no heat. 

 If so, what is it that has expelled or destroyed the light ? 



The experiments above referred to I made with the recently intro- 

 duced luminous paint. It presented the facts under their simplest 

 form. But I have also tried many other samples, for which I am 

 indebted to the courtesy of Professor Barker of Philadelphia, Among 

 them I may mention as being very well known the specimens made 

 by Dubosc, enclosed in flat glass tubes, contained in a nialiogany 

 case, and designed for illustrating the different colored phosphorescent 

 lights emitted. They are to be found in most physical cabinets. 



