36 



THE MODERN REFLECTING TELESCOPE. 



and certain when the methods of testing and figuring described in the preceding 

 chapter are used, that I have no hesitation in saying tliat when a large paraboloidal 

 mirror of short focus and of the finest attainable figure is to be made, it is economi- 

 cal to make a plane mirror of the same size, with which to test it, if one is not 

 already available. The concave mirror is first figured spherical and is used thus for 

 testing the plane mirror while the latter is being figured; the plane mirror is then 

 used in testing the concave one during the parabolizing of the latter. Both the 

 plane and paraboloidal mirrors are then used in testing the small (convex) 

 hyperboloidal mirror wliile the latter is being figured. 



Testing a Paraboloid on a Star. With this method the miri-or sui-face, as seen 

 with the knife-edge test, presents the same genei'al appearance as in testing in con- 

 junction with a large plane mirror; in the latter test, however, errors of surface are 



Fig. 13. Full-Size Polishing Tool for Parabolizing. 



seen in greater relief, because the effect of such errors is doubled on account of the 

 two reflections from the paraboloid. In addition, it is impossible to overestimate the 

 advantage of being able to test as often as is desired, in the optical laboratory, 

 where atmospheric and tempei-ature conditions can be controlled perfectly, and 

 where the mirror does not have to be removed from the polishing machine in order 

 to test it. In testing on a star it is seldom indeed that atmospheric conditions are 

 sufficiently fine to allow any except the larger errors of surface to be seen. 



Changing a Spherical Surface to a Paraboloid. As before stated, this is ac- 

 complished by shortening the radii of curvature of all of the inner zones of the sur- 



