The ink composition comprises as its coloring matter component a water-based magenta composition containing anthrapyridone compound represented by the formula : wherein R represents hydrogen atom or methyl group; X represents phenyl group, phenoxy group or halogen atom; and n is an integer of 1 to 4, or salt thereof. In the process for inhibition of polymerization of a acrylate ester; an N-oxyl compound and at least one member selected from the group consisting of manganese salt compounds, copper salt compounds, 2,2,6,6-tetramethylpiperidine compounds, and nitroso compounds are used together as polymerization inhibitors, and further an acid is used. The elastic member includes an upper elastic member disposed at an upper side of the lens holder. Each of the unobstructed planar portions contains an amount of radiation which is representative of a respective geometric dimension of the part.