Abstract: This invention provides a method of observing the interior of a furnace by viewing, at a remote location, the image depicted by a pinhole camera sited in or within the furnace housing. The image may be real and viewed directly or via a reflective or fibre optic transmission system, or alternatively a video camera may be employed, in this instance the `screen` of the camera may be constituted by light responsive elements, e.g. charge coupled devices. The camera may typically be sited in the furnace wall or roof and a pressurized blast of air or an inert gas, e.g. nitrogen or argon, may be employed for maintaining the integrity of the pinhole.