Wavefronts can be corrected using many of the same mechanisms that cause wavefront distortion. An image from a distorted mirror can be corrected by distorting another mirror into the proper shape to compensate for the wavefront variations. Local variations in the refraction index that were the cause of an aberration can be reversed, or conjugated, to provide the correction. Corrective lenses for eyes use this principle. The aberrations due to a malformed or strained lens in the eye are removed by placing an external lens in the path of the light entering the eye. The method of corrective eyeglasses is modal and static. Usually, only focus (curvature of eld) and astigmatism are corrected and they do not have any active capability. This, then, is technically not adaptive optics; however, correction to the wavefront on most adaptive optics systems is based on the principle of adding a correcting device to the beam of light. When the correction is made active, that is, by moving the corrective element, it is said to be “inertial,” with the correction linearly proportional to the stimulus.