hyperbolide

— It is shown, in conclusion, that with the Cassegrain telescope the best definition is obtainable when the form of the primary mirror is a paraboloide, and that of the secondary a hyperbolide; that deviations from these forms in the two surfaces when acting in combination, will generally tend to compensate each other, and will be entirely neutralized when the amount of error in the convex mirror is double that of the concave mirror.