Abstract: A variable power optical system composed of a first optical part, a second optical part, and a third optical part, with the first optical part being movable with respect to the second and third optical parts to change the focal length of the entire system. The first optical part is movably held between the second optical part and the third optical part. The first optical part moves in the direction perpendicular to the optical axis over a distance between the position on the optical system of the entire system and the position of retraction from the optical axis of the entire system.

Abstract: A variable power optical system composed of a first optical part (G1) for refraction, a second optical part (G2) for reflection or transmission, and a third optical part (G3) for refraction, with the first optical part being movable with respect to the second and third optical parts to change the focal length of the entire system, wherein the second optical part or the third optical part has a guide 4 which is in contact partly with the first optical part in such a way as to restrict its moving direction and the first optical part is movably held between the second optical part and the third optical part. This structure eliminates the necessity of separately forming the guide to move G1. The guide 4 is formed from a synthetic resin integrally with the stationary optical part (G3).

Abstract: A variable power optical system composed of a first optical part (G1) for refraction, a second optical part (G2) for reflection or transmission, and a third optical part (G3) for refraction, with the first optical part being movable with respect to the second and third optical parts to change the focal length of the entire system, wherein the second optical part or the third optical part has a guide 4 which is in contact partly with the first optical part in such a way as to restrict its moving direction and the first optical part is movably held between the second optical part and the third optical part. This structure eliminates the necessity of separately forming the guide to move G1. The guide 4 is formed from a synthetic resin integrally with the stationary optical part (G3).

Abstract: A core bit includes a tube (2) having an opening end surface (21) in the axial direction and a plurality of tips (3) fixed to the opening end surface (21) of the tube (2). Each of the tips (3) includes an abrasive grain layer (31) that contains diamond abrasive grains and a binder for bonding the diamond abrasive grains to each other. The diamond abrasive grains contain not more than 0.03 weight % of inclusions. The inclusions contain iron and nickel as main components. The binder contains at least 0.1 weight % and not more than 2.0 weight % of graphite. The core bit has a short perforation time, i.e., a high cutting speed, excellent sharpness, excellent durability, and a long life. The core bit is suitable for boring a concrete structure.

Abstract: A collapsible basket for receiving printed boards. This basket comprises a pair of side plates on which a plurality of ribs are formed to define grooves for receiving the printed boards, and a pair of hinged end plates along with which the side plates define a storage space for the printed boards. A supporting member, one end of which is pivotably supported on one of the side plates, is positioned vertically when the basket is collapsed for storage and is positioned horizontally, such that the free end engages the opposite side plate to retain the side plates securely spaced from each other by a preselected distance for reinforcing the basket when assembled for use.