THE MODERN REFLECTING TELESCOPE. 43 



three or four years. The face is usually silvered two or three times a year, to keep 

 it ill the finest condition for photography, in which any yellowing of the film is 

 very objectionable. 



CHAPTER XVI. 

 A SUPPORT-SYSTEM FOR LARGE MIRRORS. 



The proper support of mirrors in their cells when in use in the telescope is a 

 matter of vital im[)ortance. Small mirrors can be made very thick and can be sup- 

 ported at their edges as a lens is su[)p()rted ; the cell must be so designed that no 

 sensible change of position of the miri-or in its cell can occur. The necessity of 

 supporting large mirrors iu such a manner as to prevent flexure from their own 

 weight, in all positions which can occur in use, has long been recognized, and elab- 

 orate su[)port-systems for this purpose have been devised and used byRosse, Grubb, 

 Common, and others. Comparatively little attention has been given, however, to 

 two additional requirements which are no less important ; first, the position of the 

 mirroi-s in their cells should be defined with the greatest attainable stability, in 

 order to secure permanence of adjustment or collimation ; second, the method of 

 support should be such that the silvered back of the mirror is exposed to the air as 

 freely as possible. It is assumed that a large miri'or need never be turned farther 

 than ninety degrees from the position in which it lies horizontal upon its back. 



In the Astrophysical Journal for Fel>ruary, 1897, the writer described a 

 method of supporting large mirrors which fulfills all of the requirements named in 

 the preceding paragraph. I have employed this method in the designs for the 

 support-system of the 5-foot mirror. These designs are described and illustrated 

 here. 



I. — The Bach-Support. 



Let us consider the miri'or to be divided into twelve imaginary segments 

 of ecjual weight, as shown in Fig. 16, Plate viri. The back of the miri'or rests, pri- 

 marily, upon three strong bronze plates, each ten inches in diameter, represented by 

 the double circles a Fig. 16 and at a Fig. 17, the center of each plate being exactly 

 behind the center of weight of the corresponding segments ; these are called the 

 stationary [)lates. The upper surface of each plate is flat and is ground to fit the 

 flat back of the glass; the lower sui-face is spherical, and is ground to fit the lai-ge 

 spherical socket in which it rests. It will be noticed that these plates are near the 

 edge of the mirroi', in the outer ring of segments; the base of stable support is 

 therefore large. It is evident that by properly designing these plates and their sup- 

 ports we can fix with very great stability the [ilaneof the nu'rror which I'ests directly 

 upon them ; there is no building out from the thiee primary i)oints of support by 

 means of intermediate levers and triangles, as in the older systems. 



The weight of the remaining nine segments of the mirror is just balanced by 

 means of nine weighted levers, each of which is entirely independent of every 



