of a Solar Spectrum. 163 



to increase toward the red ray, and when flint-glass or rock 

 salt-prisms are employed to give its maximum far beyond 

 that ray, it was supposed that this heat expelled the light, and 

 consequently in all those parts of the phosphorus on which 

 it fell the surface became dark through the expulsion or 

 exhaustion of the light. 



I speak of this as " the old view," because, as I have else- 

 where shown, the curve supposed to represent heat, light, and 

 actinism, so called, have in reality nothing to do with those 

 principles. They are merely dispersion-curves having relation 

 to the optical action of the prism and to the character of the 

 surface on which the ray falls. (Phil. Mag., August 1872, 

 December 1872.) 



But this heat explanation of the phosphorescent facts cannot 

 be applied to the photographic. Nothing in the way of 

 hastened or secondary radiation seems to take place in that 

 case. 



In phosphorescence the facts observed in the production of 

 this blackness are these. If a shining phosphorescent sur- 

 face 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 surrounding luminous parts. If now 

 we no longer admit any spectrum-light, but watch the phos- 

 phorescent 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 sur- 

 rounding parts in the course of some hours slowly overtake 

 it, emitting the same quantity of light that had previously 

 been expelled from it ; and eventually all becomes dark. 



Now, apparently, all this is in accordance with the hy- 

 pothesis 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 



