DYEING AND OTHER PROCESSES OF COLOURING 3C9 



(For simplicity, the reagent made from pararosaniline instead of 

 basic fuchsine is shown here; see p. i6o). Schiff's reagent is not 

 itself a dye, for it lacks a chromophore and is therefore colourless. 

 It is often regarded as a leucobase, but this is an error; for a 

 leucobase becomes coloured on oxidation and could not possibly 

 serve in Feulgen's reaction, which is not a test for oxidizing agents. 

 When Schiff's reagent comes into contact with an aldehyde, the 

 chromophore of the triarylmethane dyes is reconstituted. There 

 seems to be no reason to deny the name of dye to the additive 

 compound of aldehyde with Schiff's reagent. Since one treats the 

 tissues with a colourless substance and a dye is formed in them, 

 there is some resemblance to the use of indigo and the azoic dyes 

 in the textile industry; but there are also important differences. 

 With the textile dyes it is necessary to provide from outside the 

 fibre both the constituents required for the making of the dye 

 (indigo-white and oxygen, or naphthol and diazo-compound), 

 while the tissues provide one of the reactants (aldehyde) when 

 Schiff's reagent is used. 



Another important difference is that the Schiff/aldehyde com- 

 pound need not be insoluble. If one adds soluble aldehyde to 

 Schiff's reagent in a glass vessel, one may then use the soluble, 

 coloured reaction-product as a dye : it acts like other basic dyes in 

 colouring acidic objects. *^^'*^^ Used in this way, the dye has 

 naturally no affinity for aldehydes. The possibility exists that 

 Schiff's reagent may form a coloured dye in the tissues by com- 

 bination with aldehydes, and the dye may then wander off and 

 colour any acidic objects. ^^" Feulgen and Rossenbeck's ^^* re- 

 action for DNA relies on the visualization by Schiff's reagent of 

 an aldehyde produced from deoxypentose. The exact position 

 of DNA can only be revealed by this test if the reaction-product 

 remains at the site of its formation. 



When we use dyes in ordinary microtechnique, we soak the 

 tissue in a coloured fluid and then observe the distribution of the 

 colour among the tissue-constituents. To do this w^e shine visible 

 light through the object and rely on the chromophores of the 

 attached ions to lower the amplitude of rays of certain wave- 

 lengths. Our colour-sense is thus stimulated. That is generally the 

 purpose of dyeing. 



Sometimes, however, we use what are unquestionably dyes and 

 allow them to become attached to the tissues by a process that 



