Witt in 1876 toformulate his " chromophore-auxochrome " theory.
On this theory colour is regarded as due to the presence of a " chromophore," and dyeing power to an " auxochrome "; the latter by itself cannot produce colour or dyeing power, but it is only active in the presence of a chromophore, when it intensifies the colour and confers the property of dyeing.
Also the relative position of the auxochrome to the chromophore influences colour, the ortho-position being generally the most powerful.
The method is based on the supposition that the magnetic rotation measures the strain produced in the molecule by an auxochrome, and he arranges the groups in the following order: O.
On the chromophoreauxochrome theory (the nitro group being the chromophore, and the hydroxyl the auxochrome) it is necessary in order to explain the high colour of the metallic salts and the colourless alkyl and aryl derivatives to assume that the auxochromic action of the hydroxyl group is only brought strongly into evidence by salt formation.