50 THE JIODERN REFLECTING TELESCOPE. 



employed when the telescope is used as a Cassegrain and as a coude respectively ; 

 each carries a convex mirror 19 inches in diameter and 3^ inches thick, of the finest 

 optical glass, and of the proper curvature for the purpose desired. 



Fio-s. 24 and 25, Plate xri, show the telescope used as a Cassegrain. In these 

 cases the amount of amplification introduced by the convex miri'or is about 3|^ 

 diameters (see p. 38) ; the equivalent focal length is therefore about 87^ feet, 

 and tlie ratio of aperture to focal length as 1 to 11 \. Fig. 24 shows the telescope 

 as used for dii-ect photography with the doul)le-slide plate-carrier at the secondaiy 

 focus. In Fig. 25 a spectrograph similar to the large Bruce spectrograph of the 

 Yerkes Observatory is shown attached to the north side of the short cast-iron sec- 

 tion of the tube ; this affords a most stable base of support for the spectrograph, 

 at a point where it can be easily counterpoised. 



Fiffs. 26 and 27, Plate xir, illustrate the use of the telescope as a coude; the 

 curvature of the ccmvex mirror is now such that the equivalent focal length is 

 about 125 feet. The cone of rays from the convex mirror strikes a diagonal plane 

 mirror at the intersection of the polar and declination axes, and is by it reflected 

 in a constant direction, which can be toward either the north or south pole of the 

 heavens, as desired. This ari'angement is almost indispensable when exti-emely 

 large and powerful spectroscopes and other kinds of physical apparatus are to be 

 used with the telescope ; the focus is now in a constant position, so that such 

 instruments need not be attached to the telescope, but can he mounted on station- 

 ary piers, in constant tt^mperature rooms, if desired. 



A brief descri[)tion of the mechanism for quick-motion and slow-motion in 

 right ascension and declination should be given. These are planned to be entirely 

 electrical, although hand-motions are added, to be used in case of an emergency. 

 Quick-motion in right ascension, both east and west, is given by the reversible 

 motor to; this is connected by geai'ing to the large bevel-gear x through the 

 medium of an electric clutch y. The bevel-gear X is permanently fixed to the 

 polar axis. When the switch whicli starts the motor is thrown in, the electric 

 clutch ij acts, and a motion of rotation is communicated to the polar axis; this 

 rotation is only at the late of 45 degrees per minute; this is sufficient, since 

 reversal is never necessary ; hence very little power is i-equired. The clutch is so 

 adjusted that it will slip when even slight undue resistance is encountered. When 

 the current is shut off from the motor the clutch is i-eleased automatically; the 

 polar axis is then fiee from the motor and gear-train. 



Quick-motion in declination is given in a manner entirely similar to that in 

 right ascension, by a small I'eversible motor attached directly to the large cast-iron 

 fork; this motor drives, through the media of a gear-train and an electric clutch, 

 the toothed sector z, which is pei'manently fixed to the cast-iron section of the tube. 

 The driving-clock and 10-foot worm-wheel are "clamped in" to the polar axis, 

 when desired, by the electric clamps /' which lock the 10-foot woi-m-wheel to the 

 bevel-gear x ; the former is of course free to turn on the polar axis when not thus 

 clamped. 



Slow-motion in right ascension is given by means of a small reversible motor 



