We report on the design of a planar solid immersion lens which is theoretically capable of confining light to a focal spot with full width half maximum of 0.29 λ (154 nm at λ= 532 nm) and achieving an effective numerical aperture of 1.732. The design is a single element device and is based on the well-known Cartesian oval. Devices are fabricated and optical characterization is performed using a far-field imaging technique providing a lower bound on the numerical aperture of 0.7. Experimental results compare well with two dimensional finite element method simulations. Devices based on the Cartesian oval can be mass produced using ultraviolet lithography and produce focal volumes lower than expensive microscope objectives.