Spiropyran compound

A salt of a spiropyran compound and an acidic compound, the spiropyran compound being represented by the formula ##STR5## wherein R.sup.1 means a C.sub.1 to C.sub.20 alkyl group, an aralkyl group, a hydroxyethyl group, an acryloxyethyl group or a methacryloxyethyl group, R.sup.2 and R.sup.4 are the same or different and each mean a hydrogen atom, a C.sub.1 to C.sub.6 alkyl group, an aryl group, an aralkyl group, a C.sub.1 to C.sub.5 alkoxy group, a halogen atom, a cyano group, a trichloromethyl group, a trifluoromethyl group or a nitro group, R.sup.3 and R.sup.5 are the same or different and each mean a hydrogen atom, a C.sub.1 to C.sub.6 alkyl group, an aryl group, an aralkyl group, a C.sub.1 to C.sub.5 alkoxy group, a halogen atom, a cyano group, a trichloromethyl group, a trifluoromethyl group, a nitro group, an amino group, a dimethylamino group or a diethyl amino group, R.sup.6 and R.sup.7 are the same or different and each mean a hydrogen atom, a C.sub.1 to C.sub.6 alkyl group, an aryl group, an aralkyl group, a halogen atom, a cyano group or a nitro group, R.sup.8 means a hydrogen atom, a vinyl group, a group --CH.sub.2 OR.sup.9 or a group --CH.sub.2 OCOC(R.sup.10).dbd.CH.sub.2 (wherein R.sup.9 means a hydrogen atom or a C.sub.1 to C.sub.4 alkyl group and R.sup.10 means a hydrogen atom or a methyl group), and X means an oxygen atom or a sulfur atom.