DEVICE AT AN INSTRUMENT FOR OPTICAL PREFERABLY VISUAL DETERMINATION OF A DEFINITE PLANE

摘要

A parallelepiped box is provided with three screens of parallel opaque lines, forming a definite plane, separated by transparent interspaces, and with a mirror behind the screens to direct ambient light from an opening in the top of the box through the screens. A forward screen is provided in the front of the box. Two rearward screens are behind the forward screen with one screen at an angle with the forward screen, the angle being in the definite plane formed by the parallel lines. Light enters the box through a diffuser in the opening and is directed through the screens to produce moire patterns indicating whether an observation is made above, below or in the definite plane. A second mirror may be provided above the opening to direct ambient light thereinto or a light source may be located above the opening to provide light thereto.