THE MODERN REFLECTING TELESCOPE. 49 



assist in giving smoothness and accuracy of driving; this worm-wheel is 10 feet in 

 diameter. 



Attached to the uppei' surface of the 4-foot head of the polar axis, by means of 

 a cii'cle of 2-incli bolts, is the lai-ge cast-iron fork u, diffei'ent views of which are 

 shown in Plate xi and Fig. 22, Plate xn. The extreme outside width of this fork 

 is 8^ feet; it is of hollow or box section, with walls averaging 1^ inches thick ; it 

 weighs about five tons. 



Between the two arms of the fork hangs the short I'ound cast-iron section v of 

 the tube; two 7-inch steel ti'unnious, having large heads or flanges, are bolted to 

 this casting, and turn in bronze bearings at tlie upper ends of the fork arms; this 

 part of the tube is 46 inches long; its inside diameter is 70 inches; its thickness is 

 1 inch ; it is reinforced at top and bottom by flanges. To the lower flange is con- 

 nected the cell-plate (desciibed in the preceding chapter) which carries the large 

 mii'ror and its support-system. 



To the up[)er flange of the short cast-iron section of the tube is bolted a strong 

 cast-ii'on ring which forms the lower end of the main or permanent section of the 

 octagonal skeleton tube; this section is 13 feet 11 inches long, and 6 feet 8 inches 

 outside (diagonal) diameter. It is constructed of eight 4-inch steel tubes, connected 

 by strong rings designed to I'esist compression ; diagonal braces, which are connected 

 together at all intei'sections, greatly inci'ease the I'igidity of the structure. This 

 entire section is so rigid that it can be placed in a large lathe for facing the ends 

 parallel to each other, and for tui'uing a slight i-ecess in the ends for the purpose of 

 accurately centering the parts which are to be connected to them. 



To the up[)ei' end of the permanent section of the skeleton tube can be attached 

 any one of three short extension tubes oi- frames, as desired ; two of these are shown 

 in Plate xi. The lower end of each extension is turned true, with a projecting ring 

 which fits into the turned recess in the uppei" end of the permanent section. With 

 this arrangement the various extensions can be removed and replaced without sen- 

 sibly affecting the adjustments of the mii-rors and othei' apparatus which they carry, 

 with reference to the optical axis of the large mii'ror. 



The extension which is shown in place on the telescope in Plate xi and in Fig. 

 23, Plate xn, is the longest one ; it is 6 feet 11 inches long ; it is used for all work 

 at the primary focus of the telescope; it carries the diagonal plane mirror and its 

 supports, and the eyepiece and double-slide plate-carrier. This extension can be 

 rotated upon the turned end of the permanent section, so that the eyepiece or pho- 

 tographic apparatus can be brought to the side of the tube which is most con- 

 venient for observing or photographing a given object. The diagonal plane mirror 

 is of the finest optical glass, is elliptical in outline, is 15 x 22 inches in size, and is 

 3^ inches thick; it is cariied in a strong cast-ircm cell, which is sup[)orted from tlie 

 skeleton tube by four thin steel plates, as shown in Plate xi. The diagonal plane 

 mirror is sufl5ciently large to fully illuminate a field 7 inches in diameter at the 

 primary focus. The double-slide plate-carrier is designed for 6.^ x 8^ inch photo- 

 graphic plates. 



The other two extensions of the tube, which are only about 2 feet long, are 



