284 Messrs. R. G. Parker and A. J. Dalladay on a 



will appear owing to different contraction as they cool, now 

 as one piece, but for some purposes it may not matter. Such 

 strain as this cannot be obviated by " annealing/'' Difference 

 in the annealing-points is the more exacting factor, for unless 

 the annealing-points of the glasses to be joined are fairly 

 close (within perhaps 50° C.) the more fusible one will reach 

 its annealing-point before the less fusible reaches a tempe- 

 rature at which it will unite with another glass in any 

 reasonable time. The annealing-points of glasses in common 

 use may differ by 200° C. or even 250° C, and it is obvious 

 therefore that many pairs of glasses could be selected which 

 would be very difficult or impossible to join ; on the other 

 hand, for most purposes suitable glasses can be chosen. 

 With regard to the last factor, it appears that certain kinds 

 of glass which will not fuse together readily in the blowpipe- 

 ffime will also not unite by the new method. 



A Payleigh cell prepared o£ hard crown glass with plate- 

 glass windows was heated to a low temperature — 400° G. — 

 at the rate of 2°-2^° G per minute and kept at the maximum 

 temperature for 1^ hours. Pressure was not applied. In 

 consequence of this, and also of the different temperature 

 expansion coefficients, they remained in optical contact only 

 over a very small part of the surfaces. When cold the glass 

 was found to have united in these places in spite of the un- 

 necessarily low temperature. The areas of contact were so 

 small that the windows could be forced off without fracture, 

 and they then carried with them glass from the other 

 component. The optical surfaces had not been changed by 

 the heating. 



This experiment (as well as that described above) indicated 

 that two glasses of quite widely differing annealing-points 

 could be joined even under unfavourable conditions. 



A more severe test was then carried out with an object- 

 glass having curves of the same radius on the common faces 

 of its components. The annealing-points of the two glasses 

 employed (flint and crown glass) differ so widely, it was 

 thought advisable to heat approximately to the annealing 

 temperature of the softer glass. The lenses, in optical contact, 

 were clamped together at the edge at three equidistant points, 

 then heated at l°-j^° G per minute to a maximum tempe- 

 rature of 476° C. They remained so for 1 hour and then 

 cooled at 4° C. per minute. They were then completely out 

 of contact, except under the three points at which pressure 

 had been applied ; in these places there was perfect union. 

 The flint lens had softened to such an extent that slight 

 dents had been made in its surface by the clamp. Both 



