160 Dr. J. W. Draper on the Phosphor ograpk 



For this reason also, the bright rectangle in a phosphoro- 

 graph oi' the solar spectrum, arising from the coalescence of 

 the infra-red lines a, ft, 7, is never sharp on its edges. It 

 seems as if it were fading away on either side. It is also 

 broader than would correspond to the actual position and 

 width of those lines ; and, particularly, it is somewhat rounded 

 at its corners. 



If we could obtain a thermograph of the solar spectrum, 

 it would correspond very closely to the phosphorograph. The 

 particles heated would radiate their heat to adjacent ones. 

 Nothing like sharpness of definition could be obtained, except 

 in very brief exposures before the effect had had time to 

 spread. 



IV, Examination of Phosphorescent Tablets by Gelatine 

 Photography . 



The examination of a phosphorescent surface can be made 

 now in a much more satisfactory manner than formerly. The 

 light we have to deal with, being variable, declines from the 

 moment of excitation to the moment of observation ; and, 

 though the phosphori now prepared are much more sensitive 

 and persistent than those formerly made, they must still be 

 looked upon as ephemeral. To examine them properly, the 

 eye must have been a long time in darkness to acquire full 

 sensitiveness. 



It was recommended by Dufay to place a bandage over 

 one eye, that its sensitiveness might not be disturbed, whilst 

 the other, being left naked, could be used in making the 

 necessary preparations. But this on trial will be found, 

 though occasionally useful, on the whole an uncomfortable 

 and unsatisfactory method. 



The exceedingly sensitive gelatine-plates now obtainable 

 remove these difficulties. The light emitted by blue phosphori, 

 such as luminous paint, consists largely of rays between H 

 and G ; and these are rays which act at a maximum on the 

 gelatine preparation. So if a gelatine plate be laid on a 

 shining blue phosphorus it is powerfully affected, and any 

 mark or image that may have been impressed on the phos- 

 phorus will on development in any of the usual ways be 

 found on the gelatine. The gelatine has no need to wait 

 after the manner of the eye. It sees the phosphorus instantly. 

 It is impressed from the very first moment ; and whilst the 

 eye is accommodating itself and so losing the best of the. effect, 

 the gelatine is gathering every ray and losing nothing. 

 Moreover the effect upon it is cumulative. The eye is 



