Abstract: An adjustable support for an optical or other instrument (e.g. a binocular microscope for use by a surgeon in neurosurgery) which readily permits adjustment in the position of the instrument in a predetermined spatial envelope and/or adjustment of the orientation of the instrument at any given position within that envelope, and which is compact, relatively inexpensive to manufacture, easy to adjust, and incorporates a high degree of safety, comprises a rod-like carrier, supporting the instrument at one end and an adjustable counterweight at the other, mounted, by a ball joint arrangement on a linkage system rotatably supported on a column. The ball joint arrangement and the other joint arrangements in the linkage system are provided with clamping devices, normally urged to clamping position by springs but released by the application of air pressure under control of a switch device.