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.
For spectroscopic purposes the Cassegrain form has peculiar advantages, because in consequence of the less rapid convergence of the rays after reflection from the convex hyperboloidal mirror, the equivalent focus can be made very great in comparison with the length of the tube.
(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.