ASILVEREDGLASSTELESCOPE. 7 



and crown are not in the best positions ; and Mr. Airy, in mounting the Northum- 

 berhxnd telescope, had to arrange the means for turning tlic k'nses on their common 

 axis, until the finest image was attained. In no account, however, have I found a 

 critical statement of the exact nature of the (h>formation, the observers merely 

 remarking that in sonu^ positions of the object glass there was a sharper image than 

 in otlicrs. 



Before 1 appreciateil the facts now to be mentioned, many fine mirrors were 

 condemned to be re-polished, which, had tliey been prop(-rly set in their mountings, 

 would have operatiul excellently. 



In attempting to asccn-tain the nature of deformations by pressure, many changes 

 were made in the position of tlie disk of glass, and in the kind of support. Some 

 square mirrors, too, were ground and polished. As an exainple of the final results, 

 the following case is presented: A 15| inch unsilvered mirror 1^ inch thick was 

 set with its l)est diameter perpendicular, the axis of the mirror being horizontal 

 (Fig. 8). The image of a pin-hole illuminated by a lamp was then observed to be 

 single, sharply defined, and Avith interference rings surrounding it as at o, Fig. 3. 

 On turning the glass 90 dc^grees, that is one quarter way round, 

 its axis still pointing in the same direction, it could hardly be 

 realized tliat the same concave surface was converging the rays. 

 The image was separated into two of about equal intensity, as 

 at h, with a wing of light going out above and below from the 

 junction. Inside and outside of the focal plane the cone of 

 r;iys liad an elliptical section, the major axis being horizontal Ktiect of Pressure on a Re- 



• "• 1 ] ]■ 1 i ■ 1 T ■ ti • i'li fleeting Surface. 



msule, and perpendicular outside, iiiinnig the mirror still 



more round the image grachially iinpro\"ed, until the original diameter was perpen- 

 dicular again — the end that liad l)eeii the uppermost now being the lowest. A 

 similar series of changes occurri'd in supporting the glass on various parts of the 

 other semicircle. It might l)e supposed that irregularities on the edge of the glass 

 disk, or in the supporting arc would account for the pluniomena. But two facts 

 dispose of the former of these; hypotheses : in the first place if the glass be turned 

 exactly half way round, tlie character of the image is imcliauged, and it is not to 

 be believed tliat in many different mirrors tliis could occur by cliance coincidence. 

 In the second place, one of these mirrors has been (-arefully examined after being 

 ground and polished three times in succession, and on each occasion required the 

 same diameter to be perpendicular. As to the second hjpothesis no material differ- 

 ence is observed wlietlier tlic supporting arc lielow be large or small, nor wluni it 

 is replaced by a thin semicircle of tinplate lined with cotton wool. 



I am led to believe that this peculiarity results from the structural arrangement 

 of the glass. The specimens that have served for these experiments have probably 

 been subjected to a rolling opcn'ation when in a plastic state, in order to be reduced 

 to a uniform thickness. Optical glass, Avhich ma\- be made by softening down 

 irregular fragments into moulds at a temj)erature Ix'low (liat of fusion, may have 

 tlie same difficulty, but wliether it lias a diameter of minimum compression can 

 only be determined by experiment. Why speculum metal slionld \\i\\v the same 

 property might be ascertained by a critical examination of tlie process of casting, 



