264 



CARNEGIE INSTITUTION OF WASHINGTON. 



the edge to the center, and the radius of ciir\'ature of each is determined 

 by tlie knife-edge test and compared with the theoretical value. Under 

 the best conditions of air in the testing-room it is possible to determine 

 the radius of curvature of a zone within 0.001 inch (0.025 mm.). 



The tests made at the focus of the paraboloid in combination -v^ith the 

 ()0-inch plane mirror are invaluable for detecting and correcting slight 

 zonal errors of surface. In this test any 60-inch circular area of the 

 100-inch surface can be seen at onetime, and the aim of the figuring is to 

 eliminate all high or low zones shown in relief and to bring the whole 

 100-inch surface to such a condition that all parts, as seen during the 

 test, appear perfectly flat. By a combination of both tests, especially 

 in the last critical stages of figuring, we attain a degree of certainty, 

 and consequently a degree of accuracy of surface, which can not be 

 obtained by either test alone. 



After the figuring of the mirror had been completed, a photographic 

 test by the Hartmann method was undertaken, both to check the visual 

 tests and to provide a permanent record of the surface. For this 

 purpose a large diaphragm was constructed with openings 2 inches in 

 diameter along axes making an angle of 45° with one another. The 

 photographs were taken in pairs on either side of the center of curva- 

 ture of the central zone of the mirror. Since the light source was 

 stationary, the differences in the measured radii of curvature for the 

 several zones are twice as great as in the \isual test, in which the light 

 source is moved for each zone. 



The results of the measurement of three pairs of photographs are 

 given in the accompanying table: 



When in use in the telescope, the 8-inch zone will be covered by the 

 projected area of the Newtonian and the convex mirrors. The 

 largest deviation of the observed from the theoretical focal length for 

 any of the remaining zones is 0.14 mm., or about one part in 92,000. 



