It may be applied to the open end of a reflecting telescope, either of the Newtonian or the Cassegrain construction."
P. 419) suggested the division of the small speculum of a Cassegrain telescope and the production of double image by micrometric rotation of the semispecula in the plane passing through their axis.
A third form of reflecting telescope was devised in 1672 by Cassegrain (Journal des Scavans, 1672).
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.
Fewer telescopes have been made of this than perhaps of any other form of reflector; but in comparatively recent years the Cassegrain has acquired importance from the fact of its adoption for the great Melbourne telescope, and from its employment in the 60-in.
(2) As a Cassegrain reflector, fig.
(3) As a Cassegrain reflector, for photographing the moon, planets or very bright nebulae on a large scale, as shown in fig.
(4) As a Cassegrain reflector, for use with a spectroscope mounted in place of the photographic plate, fig.