Reticles  are sometimes also called “eyepiece micrometers”. They are clear circular glass inserts with  a scale inscribed on them (see first picture below).  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.   If you have a stereo or binocular microscope, there will only be a reticle in one of the lenses.  The lens that holds the reticle must have a reticle retainer ring and the reticle must be of the proper diameter for the particular eyepiece lens.   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.  Note: Eyepiece lenses can be very different. 

On some lenses, the ring is at the outer edge.  On others, it may be deep inside or you may have to unscrew a lens housing to get to the ring.  Be certain that you are removing the reticle retainer ring and not a ring that secures the lens elements in place!  If in doubt, contact your microscope supplier.  Once the ring is removed, drop in the reticle then re-install the retainer ring.  We recommend using compressed air to blow out any dust after installing your reticle.  Also, extra care should be taken with the eyepiece lens whenever it is removed from the microscope. Reticles come in many varieties and in different diameters.  Most common is a straight line reticle (shown above).  This particular one is 10mm long with 100 divisions.  It also has a cross line at 5mm.  Other types available include plain cross line, square grids and circular patterns.  Your choice of an eyepiece micrometer depends entirely on your needs.  If your microscope supplier doesn’t have the exact one you need, check with Klarmann-Rulings.  They offer a wide selection of custom reticles.

You will have to supply the reticle diameter to be assured of one that will fit. 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.  Stage micrometers can be expensive and are also available from microscope suppliers.  A cruder less expensive method (for lower powers) is to use a clear plastic metric ruler.  When you look into your eyepiece lens, the markings will always be the same but the size of the image superimposed under them will get larger with more magnification.  So, as you change to a higher power objective lens, the represented value between marks will change proportionately.  For example, if each mark represents 0.1mm with a 1X objective lens, then with a 4X objective lens, each mark will roughly represent 1/4 of 0.1mm or .025mm., which incidentally is 25 micrometers (there are 1000 um in one milimeter).