25 



undertaken on an extended basis, with the result that a fine 

 Research Laboratory for the full investigation of optical 

 glasses was, under Government auspices, fitted up by Messrs. 

 Chance Brothers. Here, during the past year, extremely 

 successful research work has been carried out on the pro- 

 duction of new glasses, on the development of new methods 

 for testing the same, and on the investigation of their various 

 properties. 



It may be mentioned that among the glasses which have 

 recently been produced on a commercial scale, as a result 

 of work carried out in the Research Department, three have 

 optical constants rather more extreme than any previously 

 available for the use of opticians. These are a fluor crown 

 of n D 1-4785, v 70-5 ; and dense barium crowns of n D 1-5881, 

 v 61-1 and n D 1-6130, v 59-0. 



The investigation of certain coloured glasses which may 

 be required of optical quality is another branch of research 

 to which attention has been devoted, and formulae have been 

 arrived at for the production of uranium and didymium 

 glasses ; but the manufacture of these has not, of course, been 

 possible under the restrictions imposed by the then existing 

 war conditions. 



Messrs. Chance Brothers have also further equipped 

 themselves for the special testing work involved in order to 

 be able to guarantee that lens-blanks or prism blocks shall 

 be of any desired standard of homogeneity, measured by the 

 distortion in wave-lengths of the wave front when parallel 

 light is passed through the component in question. Many 

 opticians have been pleased to make use of these opportuni- 

 ties of testing optical material which is required for work of 

 specially high precision. 



DEFECTS OF OPTICAL GLASS. 



The efforts of the optical glass manufacturer to produce 

 glasses of more extreme optical qualities than those at 

 present known are chiefly limited by the occurrence of 

 devitrification and opalescence, and by lack of durability in 

 the glasses which escape these two vital changes. 



As already explained, devitrification is caused by the 

 separation of definite chemical compounds from the glass in 

 a crystalline state, usuaUy spheroidal in shape. All glasses 

 on long standing are subject to this defect, which, in the case 

 of the commoner glasses made in tanks, may appear in the 

 form of large globular masses of crystalline material up to 

 2 or 3 inches in diameter. In such unstirred liquid glass, 



