The compound microscope started from the simplest beginnings. It has evolved from its original form, which consisted of two single lenses, an eyepiece and an objective, into an optical system which may have as many as ten or more elements in the objective and frequently four elements in the eyepiece. This complexity of construction results from the necessity of correcting the errors or aberrations inherent in an optical system. The field of view of the microscope is comparatively small, but the quality of definition of high powered systems in an area near the center of the field is a prime necessity. Hence the successful design of such systems requires that the spherical and chromatic aberrations be well corrected, and it is desirable that coma, astigmatism, and curvature of field be reduced insofar as is possible. In the design of low powered systems greater stress is placed on good definition over a larger area of the field and here it is more important that good correction for coma, curvature of field, astigmatism, lateral color, and distortion be obtained.