Reticles are sometimes also called "eyepiece micrometers". They are clear circular glass inserts with a scale inscribed on them. The reticle or eyepiece micrometer sits right at the focal plane inside the eyepiece lens of the microscope and allows the investigator to make accurate measurements of specimens.
Reticles come in many varieties and in different diameters. Most common is a straight line reticle. Other types available include plain cross line, square grids and circular patterns. You will have to supply the reticle diameter to be assured of one that will fit.
Mounting the reticle is a somewhat critical operation. They are relatively simple to install but since they sit right on the focal plane, any dust on the reticle will be quite apparent when looking into the microscope. You can use a small spanner wrench or simply a very small flat edge screwdriver. Insert the blade into the notch in the ring and spin it out. Using compressed air to blow out any dust after installing your reticle is recommend.
The "actual" distance between any two marks on the reticle are a function of the objective lenses only. The best way to calibrate your reticle is to use what is called a stage micrometer. This is a slide that has tiny marks of a known dimensions inscribed on it. By making a comparison of the marks on the stage micrometer to the marks on the reticle, one can establish the actual value for each mark on the reticle.