Use this 30.5mm diameter BRESSER 10x Measurement Eyepiece to measure the size of your specimens through the eyepiece. This 10x eyepiece has a barrel diameter of 30.5mm to fit many of their microscope models and has a micrometer scale set in the view for easy use. For proper and precise calibration, a micrometer slide is required—and BRESSER has two available separately: one with 1/10mm scale and another with a 1/100mm scale.