THE theory of the mirage forms the subject of several recent papers by Prof. Antonio Garbasso. In notes contributed to the Atti del Lincei, xvi. (2), 1, 8, the author discusses the propagation of light in a heterogeneous medium, making use of the principle of least time, and considering the case of space of any number of dimensions defined by curvilinear coordinates. The space in question is supposed to be subject to the usual assumption that the square of the line-element is a homogeneous quadratic function of the differentials of the coordinates. As might be expected from the principle of least action (an analogy the applications of which to the problem are probably already known), the equations of the path can be reduced to the form of the ordinary equations of dynamics by a suitable choice of the characteristic function. The applications to the mirage itself are discussed in a paper in the Memorie of the Turin Academy, 1907. Prof. Garbasso claims that while the phenomenon has been studied both experimentally and theoretically, his present work fills a gap in the literature by establishing agreement of a quantitative character between the results of calculation and those of experiment.