PURPOSE: To permit easy and accurate focusing by detecting the position relations of the images of index light sources with right and left optical systems.

CONSTITUTION: After the light from an index light source Q is reflected by a beam splitter 3, it posses through a polygonal prism 2 for erecting image and is formed as an index light source image Q' on a specimen P by an objective lens 1. The specimen P and the image Q' pass through the lens 1, the prism 2, the splitter 3 then form a real image P' near a field stop 4 and the image is observed as a magnified virtural image through an eyepiece lens 5. In the focused state, the optical axes OL, OR intersect at one point on the specimen P and therefore the images Q', Q' are visible in perfect superposition, but in an out- of-focus state, the optical axes OL, OR do not intersect on the specimen P and therefore the images Q', Q' are visible as two separate images.