252 Dr. A. A. Common. On the Necessity of using [Dec. 10, 



II. "Note on the Necessity of using Well- Annealed and Homo- 

 geneous Glass for the Mirrors of Telescopes." By A. A. 

 COMMON, LL.D., F.R.S. Received November 18, 1891. 



In 1880 I ordered of the St. Gobain Glassworks, through their 

 London agent, M. de Grand Ry, a disk of glass for the mirror of a 

 5-foot telescope. The limit of weight imposed by the manufacturers 

 permitted a disk of about 61 inches diameter and 5 inches thickness ; 

 this was made with a hole through the middle of 10 inches, in order to 

 enable the telescope to be used as a Cassegrain telescope if required. 



Not being in a position to begin work at that time, the disk of 

 glass was left in its case, standing against a wall, at a slight angle, 

 till 1886, when it was put upon a grinding machine to be worked into 

 a mirror. With the intention of acquiring the necessary skill to 

 make a good mirror, I intended to make many mirrors of this one 

 disk by successive re-grinding and re-figuring. 



The first polishing was done before the whole surface of the glass 

 was brought down to a uniform face.. 



On inspection by Foucault's method of testing at the centre of 

 curvature, the image of a round hole was found to be very elliptical ; 

 very little attention was paid to this at the time, as it was thought 

 that subsequent work would bring all right. After many re-grind - 

 ings, in each of which a practically new mirror was made, this 

 elliptic appearance of the image persisted. Local polishing was tried, 

 to improve the figure of revolution, without success, in fact it made 

 matters worse. The telescope mounting being ready in 1888, the 

 mirror was finished as well as possible, and put into the telescope, 

 where a star could be examined, this not being possible when the 

 mirror was on the machine ; for photographic purposes the mirror was 

 fair, giving good stellar images, but for visual work, with moderate 

 powers, the definition did not come up to the required standard. 



It was of interest to find the cause of this bad image ; at first it 

 was thought that the long time the disk had stood on edge, at an 

 angle, had caused it ; in this case it might in time become less ; but, 

 after being re-made in 1889, and again in 1890, the mirror was, if 

 anything, worse than before. When polished, in certain lights broad 

 bands of colour, red and blue, could be seen in the body of the glass 

 about a foot from the edge all round, indicating in my opinion 

 much internal tension, probably due to imperfect annealing. 



Another disk was ordered in December, 1888, as soon as the first 

 had been tried in the telescope. This second disk was delivered in 

 1890 and made into a mirror, without showing in the slightest 

 manner any of the defects of the first. 



In the course of the work on this last disk, a discovery was ma 



