Abstract
In elementary geometry, or oftener in trigonometry, we speak of radian measure of plane angles; but, if we ever mention the measure of a solid angle by the included area of a unit sphere, it is a mere comment, and seems to have nothing to do with the fact that the area of a spherical triangle is proportional to its spherical excess. It is not easy for the pupil to infer, and he generally does not infer, that the spherical excess, expressed in radians, is precisely this measure of the solid angle, and, if multiplied by r2, gives the area of the triangle.
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