Abstract: The invention relates to the illumination of optical instruments such as microscopes or auriscopes by means of the light from light-emitting diodes (L.E.D.) (6, 50). Advantageously, the light from the diodes may be strobed. Examples described include the positioning of L.E.D.'s (6) at the focal point of the inverted condenser lens (2) of a microscope, and the inclusion of a plurality of L.E.D.'s (50) in the probe portion (38) of an auriscope. In the latter case, the device may also be provided with a source of frequency-controlled sound (28) channelled through the probe portion (38).