THE MODERff REFLECTING TELESCOPE. 45 



respect to its cell, in taking its position down against the edge-support; thus not 

 only is permanence of position lost, hut this tendency to lateral shift must imp.lir 

 the freedom of operation of the back-support system. 



In the present plan four metal arcs are used whicli rigorously define the posi- 

 tion of the mirror lateially. Two of these arcs {e Figs. 16 and 17, and Fig. 19), 

 adjacent to each other, are bolted down to the cell, and their inner edges are scraped 

 to fit the ground edge of the glass; these are called the stationary arcs; the other 

 two arcs (/Fig. 16 and Fig. 20), diametrically opposite the stationary (mes, exert a 

 slight pressure against the edge of the mirroi-, by means of .springs, foi- the purpose 

 of seating the mirror against the stationary ai'cs and holding it there; this pressure 

 need amount to only a veiy small percentage of the mirror's weight, for all 

 of the lateral pressure due to the weight of the mirroi' when the latter is inclined 

 is carried by a strong metal cotinterpoisirig ring of T section {g Figs. 16 and 17) ; 

 this completely encircles the edge of the mirroi-, and fits it loosely, a band of leather 

 or thick felt paper being inserted between the ring and the glass. For convenience 

 in description, imagine this i-ing to be suspended from the tube above, by means of 

 three short wires, so that if the mii'ror wei'e removed the I'ing could swing freely in 

 its own plane. The ring is pressed up against the edge of the mirror, when the 

 latter is inclined, by a system of twelve short weighted levers (h Figs. 16 and 17) 

 which hang perpendicular to the plane of the ring. These levers are suspended 

 from the cell-plate behind the ring, by means of ball-and-socket Joints, as shown 

 in Fig. 17, or prefei-ably, to reduce friction, on pivoted universal or Hooke's 

 joints. The ends of the short upper arms of these levers fit loosely into holes 

 in the I'ing ; the long lower ai-ms cany lead weights {i Figs. 16 and 17) which are 

 capable of slight adjustment. 



Assuming that the counterpoising ring weighs 400 pounds, so that the com- 

 bined weight of ring and mirror is 2400 pounds, the adjustment of the edge- 

 support levers is effected by turning the entire cell to a vertical plane, with the 

 mirror and ring removed, and adjusting each of the twelve lead weights until it 

 just balances a standard weight of 200 pounds hung on the short arm of the lever 

 at the point where this is to touch the ring. 



I regard the use of a support-system which will fulfill all of the conditions 

 mentioned at the beginning of this chapter as absolutely essential for large mirrors. 

 Only those who have tested large mii'rors and combinations of mii'roi-s in the opti- 

 cal shop, and those who have actually used lai'ge reflecting telescopes, can fully 

 appreciate the necessity of a support-system which will both support the mirrors 

 without constraint and flexure, and define their positions permanently with I'espect 

 to the tube and axes, in all positions of the telescope. These conditions can now 

 be attained easily and economically ; without them it is folly on the one hand to 

 expect good definition and successful photographs, or, on the other hand, to com- 

 plain that the reflecting telescope is subject to serious inherent difiiculties which 

 cannot be overcome. In the case of large mirrors in which the ratio of thickness 

 to diameter is not less than as 1 to 9 or 1 to 10 the support-system just described 

 floats the mirror so perfectly in all positions which can occur in actual use that no 



