22 ON THE CONSTRUCTION AND USE OF 



mirror was removed from its place, and ground upon a large piece of flat glass for 

 ten minutes, to produce a circular outline to the concavity. It was cemented with 

 soft pitch to the concave iron disk, the counterpart of Fig. 6, and again recentrcd 

 on the blanketed table &. Emeries of 3 and 20 seconds, and 1, 3, 10, 30, 60 minutes' 

 elutriation were worked on it, an hour each. The rate of cross motion was reduced 

 to 25 per minute to avoid heating, the mirror still revolving once for every three 

 cross strokes. The screw pressure of I was stopped. This produced a surface 

 exquisitely fine, semi-transparent, and appearing as if covered with a thin film of 

 dried milk. It could reflect the light from objects outside the window until an 

 incidence of 45 degrees was reached, and at night was found to be bright enough 

 for a preliminary examination at the centre of curvature. 



" The polisher was constructed in the usual way (page 12), and being smeared 

 with rouge was fastened to the table b, where the mirror had been. The latter 

 warmed in water to 120 F., was then put face downwards upon the former, and 

 the screw I so lowered as to cause no pressure. The machine was allowed to make 

 20 four-inch cross strokes per minute, and the polisher to revolve once for every 

 three strokes. The mirror being unconstrainedly supported on the polisher, was 

 irregularly rotated by hand, or rather prevented from rotating with the polisher. 

 The tendency of this method is to produce an almost spherical surface. To change 

 it to a paraboloid, it was only necessary when the glass was polished all over to 

 increase the length of the stroke to 8 inches, and continue working fifteen minutes 

 at a time, examining in the intervals by the tests at the centre of curvature. The 

 production of a polish all over occupied about two hours, but the correction of 

 figure took more time, on account of the frequent examinations, and the absolute 

 necessity of allowing the mirror to come back to a state of equilibrium from which 

 it had been disturbed when worked on the machine." I have seen a mirror which 

 was parabolic when just off the machine, by cooling over night become spherical. 

 And these heat changes are often succeeded by other slower molecular movements, 

 which continue to modify a surface for many days after. 



This correction, where time and not length of stroke is the governing agent, has 

 once or twice been accomplished in fifteen minutes, but sometimes has cost several 

 hours. If the figure should have become a hyperboloid of revolution, that is, have 

 its edge zones too long in comparison with the centre, it is only necessary to shorten 

 the stroke to bring it back to the sphere, or even to overpass that and produce a 

 surface in which at the centre of curvature the edge zones have too short a focal 

 length (Fig. 12). 



Very much less trouble from zones of unequal focal length was experienced after 

 this machine and system of working were adopted. This was owing probably partly 

 to the element of irregularity in the rotation of the mirror, and partly to the fact 

 that the surface is kept spherical until polished, and is then rapidly changed to 

 the paraboloid. Where the adjustments of an apparatus are made so as to attempt 

 to keep a surface parabolic for some hours, there is a strong tendency for zones to 

 appear, and of a width bearing a fixed relation to the stroke. 



The method of producing reflecting surfaces next to be spoken of, is however 

 that which has finally been adopted as the best of all, being capable of forming 



