| http://www.w3.org/ns/prov#value | - If the number of blocking rings is three or more, it is cumbersome to store and manage them. [0189] It is more preferable that the number of different heights of the lens holder units 13 be made larger than the number of kinds of heights of the blocking rings 14 to set the central thicknesses of the low-melting metal within a predetermined range.
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