1891.] Well-Annealed Glass for the Mirrors of Telescopes. 



that most probably explains the cause of failure of the first ; in the 

 process of polishing a certain amount of heat is produced, and it was 

 always the custom to allow the mirror to cool down for some hours 

 before testing ; it was also always considered that some slight change 

 of focal length was caused by the heat, but I was quite unprepared, 

 on testing the mirror directly after polishing, in order to determine 

 what this change amounted to, to find the enormous amount thus 

 produced. After two hours figuring with a 15-inch polisher, with 

 the face of harder resin than usual, so that the friction and heat 

 were below the average amount in one polishing, the change of focal 

 length was found to be 4 inches, that is to say, the image was made 

 4 inches further from the mirror than the usual place, or the mirror 

 had at that time a radius of curvature 2 inches greater than the 

 normal ; in the course of three hours this had disappeared, and the 

 image was produced at the normal place. As the whole trouble of 

 the first mirror was caused by opposite diameters of the same zone 

 coming to a focus in planes differing by about ^ of an inch, it can 

 easily be seen that this may have been caused by the failure of the 

 glass to contract in a perfect and regular manner ; in figuring, a 

 large amount of the work would necessarily be done on the expanded 

 glass, and it is reasonable to suppose that the figure thus given 

 would be correct while the glass was in this state, but that pti 

 cooling, unless the glass contracted regularly, this correct figure 

 would be lost. 



After the second mirror was finished the first mirror was re-ground, 

 and the polishing was done very slowly, half-an-hour in the morning, 

 and the same time in the evening, with manifest improvement in the 

 figure, but still without getting rid of all the defects. 



That the fault was in the glass there is no doubt ; the method of 

 working adopted shows, in the case of the second 5-foot, and in other 

 mirrors of 20-, 30-, and 36-inch mirrors, that a perfect surface of 

 revolution can always be obtained if the glass is good ; unless this 

 perfect surface of revolution is obtained, it is quite hopeless to expect 

 a good mirror. 



The makers could not re-anneal the-first disk, but they have under- 

 taken, in a very handsome manner, to replace it by a new one ; this 

 I hope will be as good as the second one made by them, which is as 

 nearly as possible perfect. 



