The _equatorial coudé_ is undeniably complicated and costly, but as constructed by Henry Frères it actually performs admirably even under severe tests, and has been several times duplicated in French observatories. The first _coudé_ erected was of 10½ inches aperture and was soon followed by one of 23.6 inches aperture and 59 ft. focus, which is the largest yet built. Still another mounting suggestive of both the polar telescope and the _coudé_ is due to Sir Howard Grubb, Fig. 92. Here as in the _coudé_ the upper part of the polar axis, _A_, is the telescope tube which leads into a solid casing _B_, about which a substantial fork, _C_, is pivoted. This fork is the extension of the side tube _D_, which carries the objective, and thus has free swing in declination through an angle limited by the roof of the observing room above, and the proximity of the horizon below. Its useful swing, as in the polar telescope, is limited by the dimensions of the mirror _E_, which receives the cone of rays from the objective and turns it up the polar tube to the eyepiece. This mirror is geared to turn at half the rate of the tube _D_ so that the angle _D E A_ is continually bisected. [Illustration: FIG. 92.—Grubb Modified Coudé.] In point of fact the sole gain in this construction is the reduction in the size of mirror required, by reason of the diminished size of the cone of rays when it reaches the mirror. The plan has been very successfully worked out in the fine astrographic telescope of the Cambridge Observatory of 12½ inches aperture and 19.3 ft. focal length. As in the other instruments of this general class the adjustments are all conveniently made from the eye end. The Cambridge instrument has a triple photo-visual objective of the form designed by Mr. H. D. Taylor and the side tube, when not in use, is turned down to the horizontal and covered in by a low wheeled housing carried on a track. The sky space covered is from 15° above the pole to near the horizontal. It is obvious that various polar and _coudé_ forms of reflector are quite practicable and indeed one such arrangement is shown in connection with the 60 inch Mt. Wilson reflector, but we are here concerned only with the chief types of mounting which have actually proved their usefulness. None of the arrangements which require the use of additional large reflecting surfaces are exempt from danger of impaired definition. Only superlatively fine workmanship and skill in mounting can save them from distortion and astigmatism due to flexure and warping of the mirrors, and such troubles have not infrequently been encountered. To a somewhat variant type belong several valuable constructions which utilize in the auxiliary reflecting system the cœlostat rather than the polar heliostat or its equivalent. The cœlostat is simply a plane mirror mounted with its plane fixed in that of a polar axis which rotates once in 48 hours, i.e., at half the apparent rate of the stars.