THE MODERN REFLECTING TELESCOPE. 25 



tioii of the stroke being changed after each complete revolution. Great care and 

 judgment must be used in this work, and the surface must be tested very often, 

 otherwise a wide zone will usually give place to several narrow ones. After the 

 protuberant zones have been softened down in this way the full-size polisher is 

 again used for finishing the surface. 



A large and perfect spherical mirror is an indispensable part of the equipment 

 of an optical laboratoiy, as it affords what is in my opinion the most satisfactory 

 means of testing large j)lane miri'ors. On account of the ease of rigorously testing 

 a concave spherical surface, this is the form which should be first attempted 

 by beginnei's in optical work. 



CHAPTER XII. 



GRINDING, FIGURING, AND TESTING PLANE MIRRORS. 



The making of large plane mirrors of fine figure is usually regarded as much 

 moie difficult than that of laige concave mii'i'ors. The diificulty has been, in the 

 past, largely one of testing. With a satisfactory method of testing the large plane 

 surface a^ a whole^ in a rigorous aud direct manner, the problem is gi'eatl}^ simpli- 

 fied. So far as the writer is aware, no such test has hitherto been fully developed. 

 In Montldy Notices, Vol. 48, [). 105, Mi'. Common suggests, very briefly, the testing 

 of plane minors in combination with a finished spherical mirror, and gives a 

 diagram in illusti-ation ; but no details in regard to the method aie given. This 

 method has been developed and used for many yeai-s by the writer in testing plane 

 minors up to 30 inches in diameter. When this test is used, the difficulty of mak- 

 ing a 24-inch plane mirror which shall not deviate from perfect flatness by an 

 amount greater than gp^^^^pp inch is neitliei' givater nor less than that of making a 

 good spherical miri'or of 2 feet apeitui-e and 50 feet radius of curvature, when 

 it is required that the radius of curvature shall not differ fi'om 50 feet by a 

 quantity greater than j^-^ inch. 



SPHERICAL 

 MIRROR 



ILLUMINATED 



I- 



PLANE MIRROR 



Fig. 2. DiAGRA.M Ii.LUSTRAiiNG Tksting of a I'lane Mirror. 



A spherical mirror A (Fig. 2), which should not be smallei' in diameter than 

 the plane mirror B to be tested, is figured with the utmost accui'acy, sjiecial care 

 being taken that no astigmatism, however slight, exists in it. The mirror A 

 is silvered ; B is polished and unsilvered. The mirrors may be set up as shown in 

 plan in Fig. 2, the distance cm -\- iitf being equal to the radius of curvature of A ; 

 lioth mirrors hang on edge in steel band.s as already described. The light proceeding 



