68 Cause of the Light Border in Photographs, §c. [May 25, 



and optical foci were made to coincide, and mnch better than one 

 which was corrected for the visual rays. 



It is needless to say that on any theory the light must not be too 

 bright or the exposure too long ; for we cannot have the exhibition 

 (in the positive) of a brighter border to a ground which is white 

 already. 



P.S. — Before presenting the above paper to the Royal Society I 

 submitted it to Captain Abney, as one of the highest authorities in 

 scientific photography, asking whether he knew of anything to dis- 

 prove the suggested explanation. He replied that he thought the 

 explanation a possible one, encouraged me to present the paper, and 

 kindly expressed the intention of submitting the question to the test 

 of experiment. 



I have referred to the photographic action of the more and less re- 

 frangible rays as antagonistic. This is practically true so far as the 

 explanation I have ventured to offer is concerned, inasmuch as the 

 more refrangible rays convert a salt of silver which is not developed 

 into one which is developable, while the less refrangible convert the 

 latter into one which is not developable. But Captain Abney has 

 pointed out to me that though the first and third salts cannot be dis- 

 tinguished by appearance, nor by the action of the developing solution, 

 they are nevertheless not the same, so that the two actions of the rays 

 are not, rigorously speaking, antagonistic, inasmuch as the one is not 

 strictly the reverse of the other. Thus with bromide of silver the 

 explanation of the observed phenomena, according to Captain Abney, 

 is that the undevelopable bromide is converted, chiefly by the action 

 of the more refrangible rays, into a subbromide, which is developable ; 

 and this again is converted, chiefly by the action of the less refrangible 

 rays, into an oxybromide, which is undevelopable. As however under 

 the ordinary circumstances for obtaining a good picture the action of 

 the light is chiefly of the first kind, and a much longer exposure would 

 be required to bring out prominently the second kind of action, the 

 explanation I have suggested is not virtually affected, though the two 

 actions could not be prolonged indefinitely, as in the illustrative expe- 

 riment in phosphorescence described in § 6. 



June 10. 



The Society adjourned over the Whitsuntide Recess to Thursday, 

 June 15th. 



