The ornamental design for optical measuring instrument, as shown and described.
1. Optical measuring instrument
1.1 : Perspective
1.2 : Front
1.3 : Back
1.4 : Left
1.5 : Right
1.6 : Top
1.7 : Bottom
Reproduction 1.1 perspective view (front, top and left side); reproduction 1.2 front view; reproduction 1.3 rear view; reproduction 1.4 left side view; reproduction 1.5 right side view; reproduction 1.6: top view; reproduction 1.7: bottom view; the thin parallel lines in the reproductions aim to reproduce the relief and not to illustrate an ornamentation of the products.