A SILVERED GLASS TELESCOPE. 7 



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

 berland telescope, had to arrange the means for turning the lenses 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 deformation, the observers merely 

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

 in others. 



Before I appreciated the facts now to be mentioned, many fine mirrors were 

 condemned to be re-polished, which, had they been properly set in their mountings, 

 would have operated excellently. 



In attempting to ascertain the nature of deformations by pressure, many changes 

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

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

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

 set with its best 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 with interference rings surrounding it as at a, Fig. 3. 

 On turning the glass 90 degrees, that is one quarter way round, 

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

 realized that the same concave surface was converging the rays. 

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

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

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

 rays had an elliptical section, the major axis being horizontal Effect of Pressure on a Re- 

 inside, and perpendicular outside. Turning the mirror still 



more round the image gradually improved, until the original diameter was perpen- 

 dicular again the end that had been the uppermost now being the lowest. A 

 similar series of changes occurred in supporting the glass on various parts of the 

 other semicircle. It might be supposed that irregularities on the edge of the glass 

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

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

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

 be believed that in many different mirrors this could occur by chance coincidence. 

 In the second place, one of these mirrors has been carefully 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 hypothesis no material differ- 

 ence is observed whether the supporting arc below be large or small, nor when 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 operation when in a plastic state, in order to be reduced 

 to a uniform thickness. Optical glass, which may be made by softening down 

 irregular fragments into moulds at a temperature below that of fusion, may have 

 the same difficulty, but whether it has a diameter of minimum compression can 

 only be determined by experiment. Why speculum metal should have the same 

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



