Popular Science Monthly 



909 



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 



