The Cassegrain telescope differs from the Gregorian only in the substitution of a convex hyperboloidal mirror for a concave ellipsoidal mirror as the small speculum.
This latter carries a hyperboloidal mirror, which returns the rays towards the centre of the large mirror and causes them to converge less rapidly.
Teeth of Skew-Bevel Wheels.The crests of the teeth of a skew-bevel wheel are parallel to the generating straight line of the hyperboloidal pitch-surface; and the transverse sections of the teeth at a given pitch-circle are similar to those of the teeth of a bevelwheel whose pitch surface is a cone touching the hyperboloidal surface at the given circle.
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