A dye of Formula (IIb) wherein X 1 and X 2 are selected independently of one another from one to 12 CH 2 units, one or more of which can be substituted independently of one another by a carbonyl group, an oxygen atom, a sulphur atom, a cis- or trans-CH=CH group, in which a CH unit can also be substituted by a nitrogen atom, an acetylenic C≡C group, a divalent phenyl, pyridine or thiophene radical, a divalent naphthalene radical, in which one or two CH groups can be substituted by nitrogen atoms, a divalent anthracene radical, in which one or two CH groups can be substituted by nitrogen atoms; and wherein up to 12 individual hydrogen atoms of the CH 2 units can each be substituted independently of one another and on the same C atoms by the halogens fluorine, chlorine, bromine or iodine or the cyano group or by a linear alkyl chain with up to 18 C atoms, in which one to 6 CH 2 units can be substituted independently of one another by a carbonyl group, an oxygen atom, a sulphur atom, a cis- or trans-CH=CH group, in which a CH unit can also be substituted by a nitrogen atom, an acetylenic C≡C group, a divalent phenyl, pyridine or thiophene radical, a divalent naphthalene radical, in which one or two CH groups can be substituted by nitrogen atoms, or a divalent anthracene radical, in which one or two CH groups can be substituted by nitrogen atoms; R 1 and R 2 are selected independently of one another from a carboxylic acid group (-COOH), a carboxylic acid ester group, a sulphonic acid group (-SO 3 H) or a halogen atom; R 3, R 4, R 5 and R 6 are methyl groups; and A represents an optional anion, which can balance the formal charge on the positively charged nitrogen atom, for use in colouring a basal membrane of the eye in surgical treatment of the eye or in diagnosis on or in the eye. wherein X 1 and X 2 are selected independently of one another from one to 12 CH 2 units, one or more of which can be substituted independently of one another by a carbonyl group, an oxygen atom, a sulphur atom, a cis- or trans-CH=CH group, in which a CH unit can also be substituted by a nitrogen atom, an acetylenic C≡C group, a divalent phenyl, pyridine or thiophene radical, a divalent naphthalene radical, in which one or two CH groups can be substituted by nitrogen atoms, a divalent anthracene radical, in which one or two CH groups can be substituted by nitrogen atoms; and wherein up to 12 individual hydrogen atoms of the CH 2 units can each be substituted independently of one another and on the same C atoms by the halogens fluorine, chlorine, bromine or iodine or the cyano group or by a linear alkyl chain with up to 18 C atoms, in which one to 6 CH 2 units can be substituted independently of one another by a carbonyl group, an oxygen atom, a sulphur atom, a cis- or trans-CH=CH group, in which a CH unit can also be substituted by a nitrogen atom, an acetylenic C≡C group, a divalent phenyl, pyridine or thiophene radical, a divalent naphthalene radical, in which one or two CH groups can be substituted by nitrogen atoms, or a divalent anthracene radical, in which one or two CH groups can be substituted by nitrogen atoms; R 1 and R 2 are selected independently of one another from a carboxylic acid group (-COOH), a carboxylic acid ester group, a sulphonic acid group (-SO 3 H) or a halogen atom; R 3, R 4, R 5 and R 6 are methyl groups; and A represents an optional anion, which can balance the formal charge on the positively charged nitrogen atom,