A New Telescope.
The optical parts of the telescope are three-the eye-piece, the object glass, which is twelve inches in diameter and a plain mirror of eighteen inches in diameter, set in front of the object glass. Focal length is sixteen and a half feet. The telescope tube is rested permanently upon two stone piers, one near each end, and twelve feet apart. About five feet of the length of the tube projects into the observatory building and the remainder is out of doors.
The novel and most important feature of the telescope is the mirror. This is polsed or hinged so that it may be set at whatever angle is necessary for reflecting the object of study into the telescope field; but there is this strict limit of the range of observation, that the object must be at or near to meridian. For the contemplated uses of the instrument this limitation is not regarded as a disadvantage, as the meridian position of an object is always best for observation because there the atmospheric obstruction is least. By varying the angle at which the mirror is hung, an object may be kept within the field for two hours, or even more. This motion is regulated by clockwork, which counterbalances the earth's advance through space.
By this means an instant comparison can be secured between the pole star, as a standard, and the object of investigation, which is to be classified. The polar telescope has an aperture of five inches and a focal length of about seven feet.
The glass of the two telescopes is of the finest French make. The mirror is four inches thick, for any plate would probably change its shape from expansion or flexure. The telescopic tube is jointed so as to be air-tight throughout and at the eye-piece. Its material is steel plate one thirty-second inch thick. The temperature of the room does not effect its interior and no fogging of the object glass has occurred during cold nights.
The availability of the plane mirror in practical astronomy has long been known. Prof. Winlock applied it in his experiments is solar photography. French astronomers also have used it in their "equatorial code." The new "horizontal telescope" is a much simpler and less expensive instrument and has the advantage of retaining more light and being no more liable to distortions or variations from changes of temperature than an ordinary reflecting telescope.
In beginning this series of photometric observations, about ten years ago, Prof. Pickering first made use of a horizontal telescope and mirror attachment. The instrument was called a "photometer." There was a companion telescope, also horizontal, combined with it for observing the reflected image of the polar star. The same instrumental adaptations were made for the subsequent or second series. This long experience gave confidence in incurring the expense of the larger under-taking now brought to perfection.