A stage assembly mountable on an optical microscope for orienting a sample into a desired focal position includes an X-axis plate operable for rectilinear shifting in the X-axis direction and a Y-axis plate mounted on the X-axis plate operable for rectilinear translation in the Y-axis direction. The light diffusing tip comprises a housing and a monolithic light scattering medium disposed within the housing. 27\u03bc absorption band of H2O and the 4.