Lummer, O. (Otto), 1860-1925, Thompson, Silvanus P. (Silvanus Phillips), 1851-1916 · London, New York, Macmillan and Co., 1900 · leaf 124
t he calls “ normal rays,” meaning by the term “ normal ray ” one which, starting from a point in the optical axis, and continually making indefinitely small angles therewith, passes through the system. These new data are the lateral distances h.2i from the axis at which the normal ray intersects… … Let us consider the accompanying Fig. 48, in which the path of a normal ray through a system of four refracting surfaces, the angles being exaggerated, however, for clearness. Then, since for small angles the tangent may be taken as equal to the arc, the quotient hjcr_^ gives the
Photographic optics
Internet Archive · Getty Research Institute · Public domain (published before 1931). Free to use for anything.
