Ernest Howard Farmer

Farmer became the first Head of Photography at the Regent Street Polytechnic (now the University of Westminster). He invented a solution, now known as Farmer’s Reducer, which reduces the intensity of a negative.
This reducer, invented in 1883. This negative reducer consists of sodium thiosulfate (hypo) + potassium ferricyanide. The latter is harmless as the cyanide is not easily liberated. is still in use today.
Farmers Reducer:

Solution 1:
Potassium ferricyanide 37.5 grams:
Dissolve in water to make 500ml:

Soution 2:
Sodium thiosulfate 480 grams:
Dissolve in water to make 2000ml

Just before use maker a working solution, combine 30ml solution 1 and 120ml of solution B then add water to make 1000ml:
Immerse the black-and-white negative into the working. Observe under strong light as the negative reduces. Remove when the density of the treated negative is acceptable. Wash in running water for 20 minutes. Treat with wetting agent and dry. Process can be repeated as required. Also works on black-and-white prints and can be applied with cotton swan or brush to treat a localized area.

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