326 HANDBOOK OF PHOTOGRAPHY 



is not a developing agent, while 

 OH 



1 : o-Dihydroxynaphthalene 



OH 



is in the class of slow developing agents. 



To increase their solubility, the sulphonic acid group, HSO3 is introduced into the 

 naphthalene structure. Thus 



NHo 



HSO; 



OH 



l-Amino-2-naphthol-6-sulphonic acid 



is the well-known Eikonogen whose developing action is quite similar to pyrogallol 

 while having the additional advantage of longer life than pyro. 



The introduction of two amino groups, in conjunction with a solubilizing sulphonic 

 acid group, into the naphthalene ring produces 



HO3S 



NHo 



NH. 



1 :4-Naphthalenediamine-7-sulphonic acid 



whose developing action is somewhat similar to the analogous p-phenylenediamine. 



When one — CH — group of naphthalene is replaced by nitrogen, quinoline or its 

 isomer, isoquinoline, is formed: 



Quinoline 



N^ 



Isoquinoline 



Further substitution of nuclear hydrogens of quinoline by hydroxyl or amino groups, 

 or both, yields another class of photographic developing agents. Thus 



OH 



N 



1 :4-Dihydroxyisoquinoline 



OH 



develops exposed silver halides to produce both metallic silver and the leuco form of an 

 orange-red dye which can be converted to the colored form by mild oxidizing agents. 



