The cones, about the size of a small walnut, bear spirally arranged imbricated scales which **subtend** the three-angled winged seeds.

Each of the twenty triangular faces **subtend** at the centre the same angle as is **subtended** by four whole and six half faces of the Platonic icosahedron; in other words, the solid is determined by the twenty planes which can be drawn through the vertices of the three faces contiguous to any face of a Platonic icosahedron.

We conclude that a double line cannot be fairly resolved unless its components **subtend** an angle exceeding that **subtended** by the wave-length of light at a distance equal to the horizontal aperture.

If 1 2 and 1 1 be the thicknesses traversed by the extreme rays, and a denote the width of the emergent beam, the dispersion is given by 0 Sµ 0 2 - 11)/a, or, if t i be negligible, 0 = Sµt/a (6) The condition of resolution of a double line whose components **subtend** an angle 0 is that 0 must exceed X/a.