56 ON QUARTIC SURFACES REPRESENTED BY A PARTICULAR EQUATION. 
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viz. reading horizontally or vertically, the dots show the tropes through each node, or 
the nodes in each trope. 
The plane through any ray and the nodal line is a pinch-plane of the surface, its 
point of contact being the intersection of the ray with the nodal line; and the inter 
section of each axis with the nodal line is a pinch-point of the surface, the tangent 
plane being the plane through the axis and the nodal line; the surface has thus 
4 pinch-planes and 4 pinch-points.