consequently when the knob 77 is depressed the block 135 comes into engagement with the exposed adhesive surface and becomes adhered to the blank b accordingly when the knob 77 is released the block 135 is withdrawn from its nest in the carrier 118 and the desired blocking of the blank b has been achieved without the need for removing it from the instrument it will be understood that the carrier 118 may be adapted to form an alloy block not shown on the blank b in place of the preformed block 135 when viewed from the side as in fig1 a the support 74 of the head 22 is initially seated within the arms 58 as shown in fig1 a a lens blank b which is usually of the fused bifocal type with an optically finished convex surface is positioned upon the supporting pins 54 with the finished surface up as shown in fig2 b the light 29 is then turned on by operation of switch 33 causing an image of the targets 84 and 85 and a further image of the outline of the reading segment portion s of the lens to be simultaneously visible through the window 39 the reading segment of the lens blank can then be decentered a prescribed distance either to the right or left of the optical center with respect to scale markings 91a markings to the right of center line 91c are used for decentering the segment portion of the lens blank to the right whereas the markings to the left of center line 91c are used to decenter to the left the v shaped lines 98 are then positioned using the knob 97 after having.