Abstract: An optical instrument for measuring the specific gravity of a solution, particularly the specific gravity of the acid in a lead acid battery, which incudes a base part and a transparent member, the transparent member including an incident light surface and reflector. The instrument further includes a light source arranged in the base part of the transparent member which is capable of radiating a straight-lined ray into the transparent member parallel to the axis of the member, and a photosensitive element for receiving light reflected by the incident light surface and the reflector. During use the instrument is positioned such that the transparent member is in contact with the solution whose specific gravity is to be measured.