About ten years ago I gave a course of lectures on Trigonometric Series, following closely the treatment of that subject in Riemann's \Partielle Di erentialgleichungen," to accompany a short course on The Potential Function, given by Professor B. O. Peirce. My course has been gradually modied and extended until it has become an introduction to Spherical Harmonics and Bessel's and Lam e's Functions. Two years ago my lecture notes were lithographed by my class for their own use and were found so convenient that.
