Foci in Complex Geometry

The object of this paper is to indicate the part played by the concept 'focus' in the geometry of the Gauss plane. The relation between focal properties and complex geometry is a reciprocal one. On the one hand, the method of complex representation may be utilised to elucidate the properties of real foci of curves; this is what is done in para. 1 of the paper, where a general theorem is stated, which gives immediately the position of the real foci of a curve given by a real areal tangential equation.