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 of the microscope. A mounting (100) for an ATR crystal (106) is carried on the support. The accessory includes a registration indicium which is located at a fixed position relative to the sample contacting area of the crystal. The registration indicium is preferably provided at the apex region of the crystal. The registration indicium can be used to ensure correct focussing of a sample when obtaining an image of the sample.