Try searching for "Eikonogen"...
Here's a recipe from Vogel (1904):
A)
100 g sodium sulfite crystals
1500 ml distilled water
8 drops concentrated sulfuric acid
25 g Eikonogen
B)
150 g crystallline sodiun carbonate, OR 100 g potasium carbonate
1000 ml disitlled water
For use mix 3 parts A), 1 part B), add some drops 10% potassium bromide solution.
Another one, from "Teknisk Ukeblad", 1891:
Vand...................................... 100 vægtsdele (Water 100 parts)
Eikonogen................................. 2 - (Eikonogen 2 parts)
Kaliumkarbonat........................ 8 - (potassium carbonate 8 parts)
Natriumsulfit.............................. 4 - (sodium sulfite 4 parts)