The Konus #5659 10x Eyepiece with Reticle is intended for the Biorex #5605, 5606, and 5607 microscopes.  Though the Biorex microscopes come standard with a 10x eyepiece, this eyepiece has an internal reticle which can aid in determining the size of particular elements within your observed sample.

This eyepiece is often inserted in to the 3rd optical path on the Biorex-3 models, but can be used in exchange of one of your normal eyepieces and you'll still see the reticle when you've got both of your eyes open.