Polychromic tetracyclo-spiro-adamatylidene derivatives, and polychromic lens incorporating said compounds

A photoreactive ophthalmic or plano lens having a heliochromic compound coated on a surface thereof or laminated or incorporated in the material of the lens, the heliochromic compound having the property of reversibly forming a coloured structural form on irradiation with U.V. light and returning to a colourless or paler structural form at normal ambient temperature in the absence of U.V. stimulation, the coloured form having the following structure (a) ##STR14## and being capable of conversion to its colourless or paler form (b) by formation of a central 6-membered ring, wherein ##STR15## represents an adamentylidene;

• R.sub.1 represents hydrogen, lower alkyl, phenyl, or phenyl alkyl;
• X represents oxygen or >NR.sub.2, in which R.sub.2 is hydrogen, lower alkyl, phenyl or phenyl alkyl and ##STR16## is a 2,5-, 2,4- or 3,5-dimethoxybenzo, a 2- or 3-furano, or a 2- or 3-thiopheno group, which Y groups are unsubstituted or may be substituted by one to four members selected from the group consisting of deuterium, lower alkyl, nitro, halogen, or phenyl.