THE MODERN REFLECTING TELESCOPE. 27 



errors; for the greatest refinement, therefore, the stand on which A and B are sup- 

 ported is so designed that the positions of the minors can be qnickly changed so as 

 to give the greatest accuracy in each part of tlie test. 



,ILLUniNATtD 



PLANE MIBROR 



Fig. 3. Diagram Illustrating Testing of a Plane Mirror. 



The use of an eyepiece in this test is important because it shows how fatal to 

 good definition is even a very slight convexity or concavity of a plane mil lor when 

 used in oblique positions. If f\ and /g coincide as closely as can be detected 

 with the knife-edge test (^ being free from zonal irregularities also) the re- 

 flected image of the pinhole, as seen in an eyepiece at _/, is as excjuisitely sharp 

 and perfect as if it wei'e formed by the spherical mirror A alone. But if £ is 

 slightly convex or concave the appearance of the eyepiece image is similar to that 

 which has ah'eady been described in connection with astigmatic concave mir- 

 rors ; the image is not sharp even at the best focus ; if jB is convex, the image 

 becomes elongated in a vertical direction outside, and in a horizontal direction 

 inside, of the best focus ; if ..B is concave the directions of elongation are the re- 

 verse of these. 



The preparation of grinding tools for plane mirrors is similar to that of tools for 

 concave mirrors. Thiee full-size, flat iron tools are usually made, however, all of 

 which are grooved. These are i^round too;ether with carborundum of finer and finer 

 grades, until all appear flat when tested with a carefully kept Brown and Sharpe 

 steel straight-edge of best quality. 



The plane mirror is fine-ground in the manner described for concave miri-ors. 

 It is of course a rare occuiTence to find a large plane mirror nearly optically flat 

 when it is first tested after gi'inding and polishing. jNIy large mirroi'S almost in- 

 variably come out slightly convex when first polished; this may be due in part to 

 the fact that the flat grinding tool becomes very slightly concave during the fine- 

 grinding of the glass, from being worked on top (see page 7). Slight convexity 

 of the mirror at this stage of the work is much better than slight concavity, for it 

 is much better and easier to remove a high center than a high edge, during tiie 

 process of figuring with polishing tools. 



Manual polishing with full-size tools should be em[)loyed when the mirror is 

 not too large to allow this. The polishing is begun with the normal tool shown 

 in Fisr. 4, in which the trrooves are of uniform width throui^hi>ut. After an hour's 

 polishing the mirror is tested; if it is found to be conve.v, polishing is continued 

 with the concaving tool shown in Fig. 5, in which all of the grooves are gradually 

 widened toward the edges of the tool, so that there is a pi-ogressive decrease of 

 action toward the edges of the glass; the amount of this widening must be 



