1882.] On the Cause of the Light Border in Photographs, fyc. 65 



accordingly a good deal more quickly discharged under such a solution 

 than under red glass, which along with the exciting rays absorbs also 

 a much larger proportion than the chromate of the discharging 

 rays. 



5. I will mention only one instance of the application of this 

 arrargement to the study of absorption. On placing before excited 

 sulphide of calcium a plate of ebonite given me by Mr. Preece as a 

 specimen of the transparent kind for certain rays of low refrangibility, 

 and then removing the intercepting screen from the lens, the trans- 

 mission of a radiation through the ebonite was immediately shown 

 by the production of the greenish light above-mentioned. Of course, 

 after a sufficient time the part acted on became dark. 



6. I will mention two more observations as leading on to the expla- 

 nation of the photographic phenomenon which I have to suggest. 



In a dark room, an image of the flame of a paraffin lamp was thrown 

 by a lens on to a phosphorescent tablet. On intercepting the incident 

 rays after no great exposure of the tablet, the place of the image was 

 naturally seen to be luminous, with a bluish light. On forming in a 

 similar manner an image of an aperture in the window shutter, 

 illuminated by the light of an overcast sky reflected horizontally by 

 a looking-glass outside, this image of course was luminous ; it was 

 brighter than the other. On now allowing both lights to act simul- 

 taneously on the tablet, the image of the flame being arranged to fall 

 in the middle of the larger image of the aperture, and after a suitable 

 exposure cutting off both lights simultaneously, the place of the 

 image of the aperture on which the image of the lamp had fallen was 

 seen to be less luminous than the remainder, which had been excited 

 by daylight alone. The reason is plain. The proportion of rays of 

 lower to rays of higher refrangibility is much greater in lamplight 

 than m the light of the sky; so that the addition of the lamplight 

 did more harm by the action of the discharging rays which it con- 

 tained on the phosphorescence produced by the daylight, than it could 

 do good by its own contribution to the phosphorescence. 



7. The other -observation was as follows : — The same tablet was laid 

 horizontally on a lawn on a bright day towards evening, when the 

 sun was moderately low, and a pole was stuck in the grass in front of 

 it, so as to cast a shadow on the tablet. After a brief exposure the 

 tablet was covered with a dark cloth, and carried into a dark room for 

 examination. 



It was found that the place of the shadow was brighter than the 

 general ground, and also a deeper blue. For a short distance on both 

 sides of the shadow the phosphorescence was a little feebler than at a 

 greater distance. 



This shows that, though the direct rays of the sun by themselves 

 alone would have strongly excited the phosphorus, yet acting along 



VOL. XXXIII. F 



