Reduce the dimensions of the horizontal circle and increase those of the vertical one, and we have the vertical circle designed by Ertel, and largely used in foreign observatories.
BOOK V.
THE EQUATORIAL.
CHAPTER XX.
VARIOUS METHODS OF MOUNTING LARGE TELESCOPES.
We have already gone somewhat in detail into the construction of the transit circle, which is almost the most important of modern astronomical instruments. We then referred to the alt-azimuth, in which, instead of dealing with those meridional measurements which we had touched upon in the case of the transit circle, we left, as it were, the meridian for other parts of the sphere and worked with other great circles, passing not through the pole of the heavens, but through the zenith.
We now pass to the “optick tube,” as used in the physical branch of astronomy, and we have first to trace the passage from the alt-azimuth to the Equatorial, as the most convenient mounting is called.
This equatorial gives the observer the power of finding any object at once, even in the day-time, if it be above the horizon; and, when the object is found, of keeping it stationary in the field of view. But although this form is the most convenient, it is not the one universally adopted, because it is expensive, and because, again, till within the last few years our opticians were not able to grapple with all its difficulties.
Hence it is that some of the instruments which have been most nobly occupied in investigations in physical astronomy have been mounted in a most simple manner, some of them being on an alt-azimuth mounting. Of these the most noteworthy example is supplied by the forty-feet instrument erected by Sir William Herschel at Slough.
Fig. 134.—The 40-feet at Slough.
Lord Rosse’s six-feet reflector again is mounted in a different manner. It is not equatorially mounted; the tube, supported at the bottom on a pivot, is moved by manual power as desired between two high side walls, carrying the staging for observers, and so allowing the telescope a small motion in right ascension of about two hours. Our amateurs then may be forgiven for still adhering to the alt-azimuth mounting for mere star-gazing purposes.