32 



THE MODERN REFLECTING TELESCOPE. 



taneously, the knife-edge is at the focus of the zone. One advantage of this 

 method is that it is independent of changes of focus of the eye itself ; but the 

 gi'eat advantage is that very narrow zones or ai'cs can be used. Diaphragms 

 with zonal openings ^ of an inch wide serve admirably for mirrors of 10 or 15 

 feet focal length ; indeed the width of the zones which are actually used is con- 

 siderably less than this; for, on account of diftraction, the edges of the openings 

 in the diaphragms always appear as brilliant lines, even while the illumination 

 near the center of the openings is being cut off by the knife-edge; it is therefore 

 only the illumination near the center that is used in making the comparison. 



The diaphragms which I use in this method of testing do not expose entire 

 zones, but only pairs of arcs on the right and left sides of the mirror. Fig. 7 shows 

 the diaphragm which was used in testing in this way the mirror of the two-foot 

 reflector of the Yerkes Observatory. The arcs are cut in a long and narrow strip 

 of thin metal; this is attached to the inner edges of two wooden strips, «/ these 



Fig. 7. Diaphragm Used in Testing a Parauoloidai. Mirror at i is Center 



OF Curvature. 



edges are curved so that all parts of the thin metal diaphragm are nearly in 

 contact with the curved surface of the mirror. The edges of the o[)enings are 

 bevelled so as to be exti'emely thin, and are finished dead-black. Twelve pairs 

 of arcs were used, with mean radii of 1, 2, 3, . . . 10, 11, and 11| inches. 

 The openings of these arcs ai'e i inch in width. The foci of the successive zones 

 (except those ueai- the center) can be readily determined by this means to within 



