Compounds such as 1,3-O-2,4-bis(3,4-dimethylbenzylidene)sorbitol, available from Milliken Chemical under the trade name Millad??? 3988 (hereinafter referred to as 3,4-DMDBS), 1,3-O-2,4-bis(p-methylbenzylidene)sorbitol, also available from Milliken & Company under the trade name Millad??? 3940 (hereinafter referred to as p-MDBS).