Some microscope cells are made by painting rings of marine glue upon a slide, and repeating this until the cell is deep enough Other cells are formed by cementing pieces of plate glass (with the interior removed) to the slides; whilst others, known as " sunk cells," are formed by grinding out a hollow in the slide. Others, again, are known as"tube cells," being formed by cementing a section of round or rectangular glass tube to the slide glass. These may be of any size. There are also "built-up cells," made by cementing separate pieces of glass together.