An accessory for use with a microscope arranged to carry out ATR measurements has a support (40) which can be mounted on the moveable stage (20) of the microscope. A mounting (100) for an ATR crystal is carried on the support. The mounting (100) is preferably pivotally mounted on the support (40) so that it can be pivoted from a position in which the crystal lines on the axis of the microscope to a position in which a sample carried on the support can be observed visually. The mounting is arranged such that it can be returned reliably and reproducibly to its original position with the crystal on the axis.