The same fundamental idea of freeing the observer from continual climbing about to reach the eyepiece has been carried out in various equatorially mounted comet seekers. A very good example of the type is a big comet seeker by Zeiss, shown in Fig. 87. The fundamental principle is that the ocular is at the intersection of the polar and declination axis, the telescope tube being overhung well beyond the north end of the former and counterbalanced on the latter. The observer can therefore sit in his swivel chair and without stirring from it sweep rapidly over a very wide expanse of sky. This particular instrument is probably the largest of regular comet seekers, 8 inches in clear aperture and 52½ inches focal length with a triple objective to ensure the necessary corrections in using so great a relative aperture. In this figure 1 is the base with corrections in altitude and azimuth, 2 the counterpoise of the whole telescope on its base, 3 the polar axis and R. A. circle, 4 the overhung declination axis and its circle, 5 the counterpoise in declination, 6 the polar counterpoise, and 7 the main telescope tube. The handwheel shown merely operates the gear for revolving the dome without leaving the observing chair. The next step beyond the eyepiece fixed in general position is so to locate it that the observer can be thoroughly protected without including the optical parts of the telescope in such wise as to injure their performance. One cannot successfully observe through an open window on account of the air currents due to temperature differences, and in an observatory dome, unheated as it is, must wait after the shutter is opened until the temperature is fairly steadied. Except for these comet seekers practically all of the class make use of one or two auxiliary reflections to bring the image into the required direction, and in general the field of possible view is somewhat curtailed by the mounting. This is less of a disadvantage than it would appear at first thought, for, to begin with, observations within 20° of the horizon or thereabouts are generally unsatisfactory, and the advantages of a stable and convenient long focus instrument are so notable as for many purposes quite to outweigh some loss of sky-space. [Illustration: FIG. 86.—Caroline Herschel’s Comet Seeker.] The simplest of the fixed eyepiece group is the polar telescope of which the rudiments are well shown in Fig. 88, a mount described by Sir Howard Grubb in 1880, and an example of which was installed a little later in the Crawford Observatory in Cork. Here the polar axis A is the main tube of the telescope, and in front of the objective B, is held in a fork the declination cradle and mirror C, by which any object within a wide sweep of declination can be brought into the field and held there by hand or clockwork through rotating the polar tube. [Illustration: FIG. 87.—Mounting of Large Comet Seeker.]