Abstract: The in situ visual inspection of down-hole equipment in drilling and exploration wells is effected by forming an optically clear region at the desired level and by using suitable video apparatus within the region so formed. This is achieved by introducing an optically clear liquid which is substantially immiscible with, and which has a density similar to that of the opaque liquid such as mud or crude oil which is present in the well. The density of the optically clear liquid can be controlled by blending two or more compatible components, e.g. as a liquid mixture or as a solution of a solid in a liquid.