l)ci"iMil tlu' wm-lv was not sii-iclly colllill^()^^, it liciii.L; iK'cessary 

 DCcasionalU in suspcnii i^dlisliin;;- tor considcrahlc inter\-al's (in 

 account dl' uii.suital)lc temperature conditions. 



It is nt)t possilile here to enter into a detailed description of 

 the jiTdcess of fi^urini^- the mirror. Roui^'hly s])eakin|Li" it may be 

 (h\-i(led into two stages. In llie first stage the mirmr was 

 hrdughi to a spherical figure: in the second this si^lierical fcjrm 

 uas cliarged to a ])araboloid. The second process though re- 

 quiring much less time than the first involved very great care 

 and freffuent optical tests to avoid the introduction of zonal 

 errors. The largest deviation of the paraboloid from the sphere 

 in the case of this mirror is only one one-thousandth of an inch. 

 .Ml of the optical work, with the exception of the first nnigh 

 shaping, was carried on with wooden tools of various sizes and 

 forms, and the use of rouge and distilled water as the polishing 

 material. 



After the completion of the mirror a series of photographic 

 tests was made to determine the accuracy of its figure. These 

 showed a remarkably high degree of perfection, every portion 

 of the surface having the same focal length to within one part 

 in al)out 90.000. 



A few figures may be of interest in this connection. The 

 finished mirror weighs 4^2 tons, about one ton of glass having 

 been removed in the process of shaping and figuring. Its 

 diameter is closely 101 inches, and its thickness at the edge 13 

 inches. The depth of the curve at the center is about 1^ 

 inches. The focal length of the mirror is five times its aperture, 

 or 42 feet. A direct photograph of the moon at this focus, 

 accordingly, would have a diameter of 4.4 inches. As in most 

 modern refiecting telescopes the 100-inch reflector will be pro- 

 vided with two small convex mirrors to be attached to the 

 upper end of the tube, either of which may be utilized to in- 

 crease the focal length in much the same way as telephoto 

 lenses are used in ordinary photography. With these mirrors 

 focal lengths of 134 and 251 feet may be obtained and the 

 magnification correspondingly increased. 



As soon as the optical work upon the mirror disk was fully 

 under way the design of the telescope mounting was begun. 

 In view of the great size and the immense weights involved the 

 "closed fork"' type was finally adopted. In this form of mount- 

 ing the telescope tube is hung in the center of a rectangular 

 frame of massive steel girders, the bearings providing for north 

 and south movements of the tube being built into the two side 

 members. The entire rectangle is mounted on bearings at top 

 and bottom which furnish the east and west motion of the tele- 

 scope. To relieve friction the system of mercury flotation used 

 most successfully for the 60-inch reflector is employed, there 

 being two large steel floats and corresponding mercury tanks, 



20 



