"Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: FIG. 1 shows diagrammatically the optical system employed in one embodiment of the invention; FIG. 2A is a side view of apparatus employing the optical system of FIG. 1; FIG. 2B is an end view of the apparatus of FIG. 2A; FIGS. 3A and 3B are a block diagram of the cir" . . . . .