Abstract: A series of photochromic compounds are disclosed which have the property of undergoing a reversible color change when exposed to unfiltered sunlight and reverting to their original color in the absence of U.V. light at room temperature. These compounds have the general formula: ##STR1## represents an adamantylidene or a substituted adamantylidene group;R.sub.1 represents hydrogen, alkyl, aryl, aralkyl or a heterocyclic group,X represents oxygen or > NR.sub.2, in which R.sub.2 is hydrogen, aryl, alkyl or aralkyl and ##STR2## represents an aromatic group, an unsaturated heterocyclic group or a benzannelated heterocyclic group.The photochromic compounds of the invention are useful in the manufacture of photoreactive lenses, particularly plastic lenses.