22 ON THE CONSTRUCTION AND USE OF 



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

 tt'U minutes, to produce a circidar outline to the concaA'ity. It was cemented with 

 soft pitch to the concave iron disk, the counterpart of Fig. (), and again recentred 

 on tile blanketed tahle h. 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 / was sto])ped. This produced a surface 

 exquisitely fine, semi-transparent, and appearing as if covered witli a thin tihn of 

 dried milk. Tt 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 curvatin'e. 



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

 with rouge was fastened to the table A, where tlie mirror bad 1)een. Tlie latter 

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

 the screw 1 so lowered as to cause no ))r(>ssure. The macliine was allowed to make 

 20 four-inch cross strokes per minute, and th(> polislier to re\()h-e once for every 

 three strokes. The mirror being unoonstraiu(>dly supported on the polisher, was 

 irregidarly rotated by hand, or rather iire\cnted from rotating \\ itb the ]>oUsher. 

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

 it to a paraboloid, it was only necessary when the glass was ])olisli(Ml all ()\cr to 

 increase tlie length of the stroke to 8 inches, and contiiuu' working fifteen minutes 

 at a time, examining in tlie inter\als Ity the tests at the centre of ( in\ature. 'I'he 

 prodiution of a polisli all owv occupied about two hours. l)ut the correction of 

 figure took more time, on account of the fre(|uent examinations, and tlie absolute 

 lU'cessity of allowing the mirror to come back to a state of equilibrium from which 

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

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

 And these lieat changes are often succeeded by other slower moleiular moAcineiits, 

 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 betm acconqdished in fifteen minutes, lait sometimes has cost several 

 hours. If the figure shonld b;i\e become a hyperboloid of icxobition. tli;it is, have 

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

 the stroke to bring it bac;k 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). 



"\^ery 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 Avidth bearing a fixed relation to the stroke. 



The method of producing reflecting surfaces next to be spoken of. is ho\A('ver 

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



