Page:Popular Science Monthly Volume 89.djvu/923

 Popular Science Monthly

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��account of its unwieldiness. Mechanical appliances for su|iporting and movintj heavy apparatus had not reached the per- fection of the present day, and the six- foot telescope of Lord Rosse could not be moved more than ten degrees either side of the meridian.

On account of these difficulties, interest in the reflector lapsed, except in England, where it has always been a fa\-orite. With the development of photography and its application to astronomy', its usefulness became very apparent. Within the last few decades, therefore, several large re- flectors have been built. They are no longer made of speculum metal, but of glass, on the front surface of which a thin film of silver is chemically deposited.

Different Types of Reflectors

In this country there are several large reflectors, some of which are famous for the work which has been done with them. Thus, there is the Crosslej' three-foot re- flector at the Lick Observ'atory, which was used by Prof. Keeler in photographing nebulae. The results of his labors showed that a very large number of nebulae have the spiral form shown in the accompanying illustrations. The Crossley instrument, however, was not of American origin. It was made in England and presented to Lick by its owner. Its installation was followed by the constructing and mounting of the two-foot reflector of the Yerkes Observatory, under the direction of G. W. Ritchey, who also made the mirror for the great sixty-inch instrument on Mt. Wilson and designed its mounting. A comparison of the two photographs of the same nebula (Messier loi in Ursa Major), on the pre- ceding page, taken with the two instru- ments, shows that both of them give beautiful results, but that the larger instrument has a greater wealth of detail.

There is a forty-inch reflector at the Lowell Observatory in Flagstaft', Arizona, and two others are in process of construc- tion — a hundred-inch for the Mt. Wilson Solar Obser\-atory and a seventy-two-inch for the Dominion Observatory- of Canada which is to be installed at Victoria in British Columbia.

The principles of construction are the same, whatever the size of the instrument, but the great weight of a large reflector makes the engineering problem a difficult one. The building of the seventy-two-inrh Canadian instrument mav lie taken as an

��illustration of some of the mechanical dif- ficulties to be surmounted, and the accom- panying pictures have been selected to show different stages in its progress. One shows the mounting as it was .set up in the workshop in Cleveland. The ends of the polar a.xis AA' are supported on steel castings which are boiled to the heads of concrete piers. The permanent pier erected at Victoria is shown also. The polar axis must be set parallel to the axis of rotation of the earth. In the latitude of Victoria it makes an angle of more than 48° with the horizon. To the uprights of the frame- work of the walls are attached horizontal ribs which are for the purpose of supporting the sheet metal walls. It will be noticed that they are in pairs, being fastened both to the inner and the outer edges of the upright beams. The sheathing is attached to both sets, forming thus a double wall, with an intermediate air space of at least six inches. This structure must be made extremely stout in order to bear the enor- mous weight of the dome. One of the pictures shows the building complete, up to the covering of the dome. This is furnished with a system of shutters which with the double wall permit the interior of the building to maintain an even and moderate temperature. Electric motors are used in mo\'ing the telescope and dome. These are controlled by push buttons, located on small keyboards con\enientIy placed for the observer to use. An im- ' portant part of the gearing is the clock- work, which carries the telescope with the rotation of the heavens, so that a star can be kept in the field of view as long as is desired. This must be made so that the telescope moves with absolute steadiness. The optical parts of the instrument are being made by Brashear at Allegheny. The large mirror is twelve inches thick at the edges and will weigh over four thousand pounds.

What Good is a Reflecting Telescope?

Ha\ing thus given somewhat in detail the construction of the reflecting telescope, it remains to describe the work which can most satisfactorily be done with it. First, it is used for direct photograjjln', both for recording very faint objects and for getting fine details of brighter objects, such as nebulae. This is perhaps the use which appeals most directly to the general reader. We can also get the photographic images of very faint stars, the twentieth magnitude

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