Precision Method of uniting Optical Glass. 279' 



values of the mobility of the particular glass at temperatures 

 o£T = 560°C. and ^ = 552° 0., 544°, 536° successively, we 

 find the respective values of the mobility to be 



g^ = 2 2 when T 1 = 552° C. 



= 2 2 =4 when T 1 =544°C. 

 = 2* = S when T x = 536° C. 



That is, the glass becomes twice as " soft " for every rise of 

 temperature of 8° C. Therefore, at 60° or 70° C. below the 

 annealing temperature a glass has become sufficiently hard 

 to withstand very great local pressure for short periods 

 without sensible deformation. The method of determining 

 the annealing temperature will be described at a later date ; 

 it should be known accurately for each kind of glass which 

 is to be joined, as by this means only can we obtain a sufficient 

 welding effect together with the minimum risk of injury to 

 the surfaces. 



Working at this temperature, b'0° C. below the annealing- 

 point, two pieces of glass under pressure and in optical 

 contact will unite perfectly in less than 1 hour. At the 

 same time the deformation of* the outer surfaces, caused by 

 the imperfections in the plates compressing them, is so very 

 slight that a few strokes on a polishing tool are all that is 

 necessary to restore them to the condition of optical surfaces 

 of high accuracy — or treatment of the surfaces may be 

 altogether unnecessary. It is perhaps well in practice to 

 work at a temperature 10°-20° below this maximum safe tem- 

 perature in order to ensure that no deformation shall occur. 



Methods of distinguishing between Optical Contact and 

 Complete Union. — With regard to tests For distinguishing 

 perfect union, we have already noted that the optical contact 

 of two surfaces is easily destroyed by sudden temperature 

 change, by the action of liquids, and by mechanical force; 

 when the contact has changed into complete union, they 

 cannot be separated by any of these means, and if force is 

 used the two pieces of glass break as one — not along the 

 interface. A crack or a diamond-cut spreads through the 

 two pieces as though one piece of glass, whereas if the pieces 

 are merely in ordinary optical contact, it cannot be made to 

 pass from one to the other ; this is well shown in figs. 2 and 3 

 (PI. V.). and is a most conclusive proof of the completeness 

 of the joining. 



