The aspect of the matter is that all microscope lenses of high power and large numerical aperture can have their residual aberration balance easily disturbed by an error in thickness of certain of their component lenses. It is a fact that lenses which are highly sensitive to thickness variations are also highly sensitive to a change in magnification, which can be varied by altering the position of the adjustable calibrated draw tube with which all standard microscopes are supplied. The optical glass is prepared first of all by being cut with a slitting saw into flat plates of convenient size, somewhere about an inch square. The most important monochromatic aberrations of a microscope object glass are spherical aberration and coma. The spherical aberration governs the contrast in the image and the defining power, whilst the coma to a large extent limits the size of the usable field.