This disclosure relates generally to analytical instruments for measuring one or more properties of specimens or samples to be analyzed and, more particularly, to an analytical instrument with an adjustable optical path length. An analytical instrument may include a specimen support upon which a specimen may rest and a compression plate for controllably adjusting an optical path length of the specimen between the specimen support and the compression plate. In particular, a specimen may contact both the specimen support and the compression plate such that controlling a distance of the compression plate with respect to the specimen support effectively controls the optical path length of the specimen. An analytical instrument may include collimating lenses to collimate electromagnetic energy for transmission through a specimen and converging lenses for directing electromagnetic energy transmitted through the specimen into one or more sensors.