PURPOSE:To enable the tightness of the inside of the body to be maintained surely by retractably mounting the objective lens sides of a pair of prisms respectively built into the body thereby performing controlling of focuses. CONSTITUTION:The prisms 9 built in the body are so supported as to be able to make advancing and retracting motion with respect to prisms 8 and sliding shafts 23 are suspended right under sliding rods 14 to the mounting frame 13 of the prisms 9 and are protruded to outside the body 1 by maintaining airtightness with O-rings. The protruded right and left two sliding shafts 23 are fixed altogether to a connecting piece 26. Hence, as a rotary operating barrel 6 is operated, the connecting piece 26 slides forward and backward like the arrow by way of screw rod 22 and therefore the prisms 9 cooperatively move simultaneously to right and left, thereby enabling focusing to be performed.
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