5. parallax error parallax is defined as an apparent displacement of orientation of an object viewed along two different lines of sight. in every apparatus where the volume is defined by a concave meniscus, the reading or setting is made on the lowest point of the meniscus. in order that the lowest point may be observed, it is necessary to place a shade of some dark material immediately below the meniscus. the meniscus of most liquids used in volumetric standards is concave, and the lowest point in the center of the meniscus is used to determine the correct volume reading. the meniscus formed by a non-wetting liquid, such as mercury (hg), is convex with the highest point in the center. the highest point of such a convex meniscus is used to make the reading. for opaque liquids it is difficult to observe the lowest point of the meniscus, therefore the upper meniscus is used to determine the reading. when reading any meniscus, it is very important to ascertain that the measuring device with the liquid is kept on a flat surface and the observation should be made at the eye level.