Prism design for scanning applications and illumination of microscopy sample

There is disclosed a prism used for scanning applications such as total internal reflection microscopy in which the prism is translated relative to an incident light beam. A geometry is disclosed which cancels walk of the beam footprint at the base of the prism. Walk of the beam due to irregularities in a largely planar sample surface located at the prism base are cancelled by coupling movement of the incident beam to movement of the sample in the field of view of the objective lens, for example as part of a autofocus arrangement.