tapered end of its conical point 15 engages an inclined surface 33 of the funnel shaped adjusting bore 21 this engagement causes the holder 5 to move in the direction 43 so that the focal point 41 is displaced as desired of course both the setscrew 13 and the clamping screw 11 must be loosened prior to adjusting the setscrew 9 the setscrew 13 may be used to displace the focal point 41 in the opposite direction since turning the setscrew 13 inwards causes its point 17 to engage an inclined surface 19 of bore 23 the amount of adjustment caused by the setscrews 9 13 may be changed by changing the angle of inclination between the optical axis 45 and the inclined surfaces 33 19 after the focal point 41 is accurately positioned the holder 5 may be locked in position with the clamping screw 11 tilting of the holder 5 is avoided since both of the setscrews 9 13 urge the holder 5 against the same surface 29 as discussed above the setscrews 9 13 may act against inclined surfaces 19 33 alternatively the tapers of points 15 17 may act against the end faces 51 53 of the holder 5 if the end faces 51 53 are inclined in a manner similar to the inclination of surfaces 33 19 in either case the angle of inclination relative to the optical axis should lie between 40 degrees and 80 degrees fine adjustments can be achieved by orienting the setscrews 9 13 perpendicularly to the optical axis 45 so that a large setscrew movement causes only a fine axial displacement of the holder 5 if the angle of inclination.