This is a special glass insert for the eyepiece lens of a microscope. The glass insert fits into one of the eyepiece lenses and has a central large square and within it is a smaller square. Using random fields where the RBCs are evenly distributed, the reticulocytes are counted in the large and small squares of the Miller disk but the RBCs are counted only in the smaller square. The idea behind this convention was you did not have to actually count 1000 red blood cells. You would count 100 (in the smaller square) but actually see 1000 by counting the retics in the entire square. There are variations of a formula for calculation of the retic percentage using a Miller disk. It is the number of retics counted/the number of RBCs times 9. This number then muliplied by 100 to get the retic percentage.