In optical computations on skew rays, there are various methods besides the trigonometrical ray tracing method which is generally used, but they are for special cases and more or less complicated. With respect to an optical system composed of spherical surfaces and planes, a trial is made in this paper to establish a systematic method of computations that is suited for skew ray tracing and calculations of aberration. The attempt seems successful to some extent by choosing three independent variables for the entrance ray enabling the classification of aberration terms by their orders and thereby deriving adequate variables and simple formulas for the ray tracing. The results are available to differential calculations, the method of which is left for future publication.
