Using simple ray tracinig technliques presented in Chapter 6, we demonstrate that a general ray is not focused to the position predicted by paraxial theory. The aberration displayed is spherical aberration. Two methods of measuring aberration: the use of optical path difference to characterize wavefront aberration. The transverse ray coefficients to generate a ray intercept plot. Experimental examples of all the third order aberrations are given. In addition to spherical aberration, they include coma, astigmatism, field curvature, and distortion Only two types of aberration correction are discussed, removal of spherical aberration in the Hubble Space telescope and chromatic aberration. A detailed example of chromatic aberration is given.