46 THE MODERN REFLECTING TELESCOPE. 



flexure or distortion can be detected with the most sensitive optical tests. Fur- 

 thermore, with the method of edge-support described, and in the case of the 5- 

 foot mirioi- weighing a ton, no lateral shift amounting to -j-ffVij" ^°^^ ^^^ occur when 

 the mirror is turned in extreme oblique positions. 



In Figs. 17 and 18 is shown the massive cell-plate of cast-iron which carries 

 the mirror and its support-system, and which is connected to the short cast-iron sec- 

 tion of the tube; this connection is made by means of strong adjusting screws, by 

 means of which the miiToi' and its support-system, as a whole, are adjusted for 

 collimating the mirror; these adjusting screws are shown at k, Fig. 18. Additional 

 screws are also shown at I in this figure ; these are backed out of the way when 

 collimating is being done; when this is finished they are brought into position, 

 and assist in bolting the cell-plate rigidly to the tube. As is shown in Figs. 17 

 and 18, the central part of the cell-plate, a circle altout 50 inches in diameter, con- 

 sists of open ribs or arms which allow free access of air to the silvered back of the 

 miiTor. 



When the face of the mirror is to be resilvered, the cell-plate, support-system, 

 and mirror are removed as a whole, and silvering is done in the manner described 

 in the preceding chapter, without taking the mirror from its sup[)orts or disturbing 

 the adjustments of the latter in any way. Furthermoi'e, the mirror can be taken 

 out of the telescope in this way, silvered, and replaced, without sensildy disturl)ing 

 its collimation or the position of the focal plane. When the back of the mirror 

 must be resilvered, which need not be done oftener than once in three or four 

 years, the glass must of course be removed from its support-system. 



This support-system, as described, may appear complicated and expensive; in 

 reality it is not so, foi- all of the levers, plates, etc., usetl for the back-support can 

 be exactly alike, as can also the levers used for edge-support ; even when a greater 

 number of levers than twelve are used the consti'uction is simple and economical. 



In Plate x is shown a 30-inch plane mirror supported at the back by twelve 

 plates and nine levers as described above; the mirror is shown unsilvered, so that 

 the plates are seen through 4 inches of glass. This is a [)art of the 30-inch coelostat 

 recently constructed from the writer's designs in the instrument and optical shops 

 of the Yerkes Observatory. 



CHAPTER XVII. 



A MOUNTING FOR A LARCxE REFLECTING TELESCOPE. 



In considei'ing the requirements for a modern reflector mounting for photo- 

 graphic and spectro.scopic work, the writer can probably not do better than to 

 describe the designs for the proposed mounting of the 5-foot reflector. These 

 designs are the result of experience both in optical work and in the use of the 2- 

 foot reflector and the 40-inch refractor of the Yerkes observatory in astronomical 

 photography. 



