a system for performing corneal laser surgery is shown in fig1 and is generally designated 10 . as shown , the system 10 includes a surgical laser unit 12 for directing a laser beam 14 along a beam path toward an eye 16 of a patient 18 . additionally , the system 10 includes a platform 20 for aligning the eye 16 of the patient 18 with the surgical laser unit 12 . further , a computer controller 22 is in electronic communication with the surgical laser unit 12 via an electrical cable 24 , for monitoring and controlling the laser surgery procedure . referring now to fig2 , an optical device for steering the laser beam 14 to a focal point in a medium , in accordance with the present invention , is shown and is generally designated 26 . generally , the device 26 will be an integral part of the surgical laser unit 12 . in any event , as shown , the device 26 includes a laser source 28 for generating and directing the laser beam 14 along a beam path 30 toward the eye 16 . preferably , the laser source 28 is a femtosecond laser source 28 , which is to say a laser source that generates a laser beam 14 having a wavelength of about one micron , a pulse duration in the range of about 100 - 1000 femtoseconds , and a pulse energy in the range of 0 . 1 to 100 mj . as shown in fig2 , the device 26 of the present invention includes a z - scanning apparatus 32 that is positioned on the beam path 30 for moving the focal point in a z - direction . referring for a moment to fig3 a and 3b , two alternate embodiments of the z - scanning apparatus 32 are shown . in fig3 a , a voice coil subassembly 32 ′ includes a lens 34 fixedly positioned on the beam path 30 . preferably , the lens 34 is a plano - convex lens . additionally , the subassembly 32 ′ includes a voice coil 36 having a movable , linear slide 38 that defines a longitudinal axis 40 . as shown , the longitudinal axis 40 is parallel to the beam path 30 . further , a lens 42 , which is preferably a plano - concave lens , is mounted on the linear slide 38 for movement therewith back and forth along the beam path 30 . as shown in fig3 b , an alternate embodiment of the z - scanning apparatus 32 is an active mirror 32 ″. more specifically , the mirror 32 ″ may be of the type disclosed in u . s . pat . no . 6 , 220 , 707 , entitled “ method for programming an active mirror to mimic a wavefront ” issued to j . bille . as can be appreciated by referring to fig3 b , the mirror 32 ″ comprises a plurality of individual facets , of which facet 43 is exemplary . importantly , the facet 43 may be independently moved to change the shape of the surface of the active mirror 32 ″ to alter the incoming beam 14 of light . it should be appreciated that the location of the z - scanning apparatus 32 on the beam path 30 , i . e . after the laser source 28 and upstream from the remaining optical elements of the device 26 , allows the beam 14 to pass through the center of the apparatus 32 , which is desirable when focusing the beam 14 to a focal point in the eye 16 . in addition to the z - scanning apparatus 32 , the device 26 includes a scanning mechanism 44 , which is preferably a galvanometric mirror , positioned on the beam path 30 for rotation of the mirror 44 through an angle “ θ ”. the mirror 44 has an axis of rotation 46 that is perpendicular to the beam path 30 . also positioned on the beam path 30 is a scanning mechanism 48 , which is also preferably a galvanometric mirror . as contemplated by the present invention , the mirror 48 has an axis of rotation 50 that is perpendicular to the beam path 30 and parallel to the axis of rotation 46 of the mirror 44 . as shown in fig4 , the mirror 48 is positioned to rotate through an angle of “ 2θ ”. further , a scanning mechanism 52 is positioned on the beam path 30 to be optically aligned with the mirror 48 . in the preferred embodiment of the present invention , the scanning mechanism 52 is a galvanometric mirror , positioned on the beam path 30 for rotation through an angle “ φ ”. it can be seen in fig2 and 4 that the mirror 52 has an axis of rotation 54 that is perpendicular to both the axes of rotation 46 and 50 , and that is perpendicular to the beam path 30 . structurally , the distance , “ d 1 ” ( fig4 ), between the center 56 of the mirror 44 and the center 58 of the mirror 48 , is equal to the distance “ d 2 ” between the center 58 and the center 60 of the mirror 52 . continuing along the beam path 30 , it can be seen in fig2 that the device 26 includes a relay 62 positioned downstream from both the z - scanning apparatus 32 and from the mirrors 44 , 48 and 52 . as shown , the relay 62 comprises a plurality of lenses of which lenses 64 a and 64 b , are exemplary . in addition to the relay 62 , a dichroic turning mirror 66 is positioned for directing the laser beam 14 toward the eye 16 as the beam 14 exits the relay 62 . more specifically , the turning mirror 66 is positioned sequentially on the beam path 30 after the relay 62 , and the mirror 66 is oriented at substantially a 45 ° angle relative to the beam path 30 . in addition to the dichroic mirror 66 , the device 26 of the present invention includes a microscope 68 optically aligned with the dichroic turning mirror 66 and the beam path 30 , for viewing the eye 16 of the patient 18 during the laser surgery procedure . still referring to fig2 , the device 26 also includes a focusing lens 70 positioned on the beam path 30 for focusing the laser beam 14 to the focal point in the eye 16 . more specifically , the focusing lens 70 is positioned downstream from the turning mirror 66 . as shown in fig2 , the focusing lens 70 is a lens multiplet . further , as envisioned by the present invention , the focusing lens 70 defines a central axis 72 . it can be appreciated that the relay 62 is located upstream from the focusing lens 70 for optically imaging the galvanometric mirror 52 onto the surface of the focusing lens 70 . stated differently , the net effect of the beam 14 passing through the relay 62 , prior to transiting the focusing lens 70 , is that the galvanometric mirror 52 and the focusing lens 70 are optically conjugated . in the operation of the present invention , the laser source 28 generates a laser beam 14 that is directed toward the eye 16 . more specifically , the laser beam 14 is steered to a focal point within a particular layer , or medium , of the eye 16 . as contemplated by the present invention , the medium defines an orthogonal x - y - z coordinate system , of which the coordinate system 74 in fig5 and 6 is exemplary . as can be seen by cross - referencing fig5 and 6 , the medium is the cornea 76 of the eye 16 , and the x - y plane of the coordinate system 74 is normal to the optical axis 78 of the eye 16 . as further shown in fig5 and 6 , the z - axis of the coordinate system 74 is substantially coincident with optical axis 78 . considering still further the operation of the present invention , the laser beam 14 exits the laser source 28 and travels along the beam path 30 towards the z - scanning apparatus 32 . in one embodiment of the present invention ( fig3 a ), the computer controller 22 directs the linear slide 38 of the voice coil subassembly 32 ′ to move axially along the longitudinal axis 40 a specified distance . consequently , the lens 42 , which is mounted on the linear slide 38 , moves axially as well . functionally , the movement of the focusing lens 42 relative to the stationary lens 34 causes the beam 14 to diverge or converge , depending on the direction of movement of the lens 42 . as a result of the divergence or convergence of the beam 14 , the x - y plane of the coordinate system is effectively moved along the z - axis to focus the focal point on the plane . it should be appreciated that the divergence and convergence of the beam 14 may also be accomplished by other means known in the pertinent art , such as by the use of an active mirror 32 ″. more particularly , prior to the laser beam 14 reaching the active mirror 32 ″, the computer controller 22 directs the movement of the individual facets , e . g . 43 , of the mirror 32 ″ to focus the focal point along the z - axis . after exiting the z - scanning apparatus 32 , the laser beam 14 continues along the beam path 30 toward the first galvanometric mirror 44 . as can be seen by cross - referencing fig2 and 4 , the laser beam 14 is directed toward the center of the mirror 44 . as shown in fig4 , the mirror 44 rotates , as directed by the computer controller 22 , about the axis of rotation 46 through an angle of “ θ ”. as the laser beam 14 reflects off the mirror 44 , the orientation of the mirror 44 relative to the angle of incidence of the beam 14 , produces a corresponding x - direction movement of the focal point in the cornea 76 . more specifically , the focal point moves along the x - axis of the x - y plane of the coordinate system 74 through a distance “ δx ” that is proportional to the angle of rotation “ θ ”. in concert with the rotation of the mirror 44 , the mirror 48 rotates about the axis of rotation 50 through an angle of “ 2θ ”. as the laser beam 14 reflects off the mirror 48 , the laser beam 14 is compensated to align the beam path 30 with the center of the mirror 52 , while maintaining the “ δx ” movement introduced by the rotation of the mirror 44 . in this way , the laser beam 14 reflects off the center of the mirror 52 , wherein the beam 14 is moved in a y - direction . more particularly , the mirror 52 is directed by the computer controller 22 to rotate about the axis of rotation 54 through an angle “ φ ”. consequently , rotation of the mirror 52 though an angle “ φ ” moves the focal point of the beam 14 a linear distance “ δy ” along the y - axis of the x - y plane . referring once again to fig2 , the laser beam 14 reflects off the third mirror 52 and enters the relay 62 . in the relay 62 , the laser beam 14 transits the lenses 64 a and 64 b , during which time the beam path 30 is positioned to be centered on the central axis 72 of the focusing lens 70 when the laser beam 14 is incident on the focusing lens 70 . more specifically , the laser beam 14 is focused by lenses 64 a and 64 b to optically position the y - direction mirror 52 coincident with the focusing lens 70 . as such , the mirror 52 changes the angle of incidence of the beam 14 prior to the beam 14 striking the focusing lens 70 . there is not , however , any lateral movement of the beam 14 away from the center of the focusing lens 70 as the beam 14 reflects off the mirror 52 and transits the relay 62 . as the laser beam 14 exits the relay 62 and reflects off the turning mirror 66 , the turning mirror 66 directs the beam 14 toward the focusing lens 70 . at the focusing lens 70 , the laser beam 14 strikes the center section of the lens 70 . as the laser beam 14 transits the focusing lens 70 , the laser beam 14 is focused onto the desired focal point in the cornea 76 of the eye 16 . throughout the course of the laser surgery procedure , a system operator ( not shown ) may view the eye 16 of the patient 18 through the microscope 68 which is aligned with the dichroic turning mirror 66 . while the particular beam steering system for corneal laser surgery as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated , it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims .