Re: What is Schiff's reagent?

I do not understand the sentence in my text book "Feulgen staining is based on the reaction of 2-deoxy sugars with Schiff's reagent (Dye Fuchsine is decolourised by sulfurous acid)."
What is Schiff's reagent and what is the relation between the sentence "dye Fuchsine is decolourised by sulfurous acid" with the Schiff's reagent?

Schiff's reagent is a fuschin dye that has been decolorized by the addition of a sulfonyl hydroxide group to the central carbon atom.

This relationship should be clear if you compare the structures of fuschin dye and Schiff's reagent, as below.

Fuchsin Dye

Schiff's Reagent

Here are some seminal references on the generation of Schiff's reagent that should be useful for you.

Lillie RD.
Simplication of the manufacture of Schiff reagent for use in histochemical procedures.
Stain Technol. 1951 Jul;26(3):163-5.

Chu CH.
A histochemical study of staining the axis cylinder with fuchsin-sulfurous acid (Schiff's reagent).
Anat Rec. 1950 Dec;108(4):723-45.

Alexander J, McCarty KS, Alexander-Jackson E.
Rapid method of preparing Schiff's reagent for the Feulgen test.
Science. 1950 Jan 6;111(2871):13.

Bargee JD, Delamater ED.
The Use of Thionyl Chloride in the Preparation of Schiff's Reagent.
Science. 1948 Jul 30;108(2796):121-2.