ii8 



NATURE 



[October i, 19 14 



as a flint glass in respect of its refractive index. 

 This property is of special value in the construc- 

 tion of modern anastigmat photographic lenses. 

 These researches of Schott and Abbe were fol- 

 lowed by the establishment of the Jena glass- 

 works, where the new types of glass were made 

 on a commercial scale. 



The immediate results were — 



(i) The manufacture of flint glasses containing 

 boric acid ; by the aid of these glasses it was 

 possible to make three-lens objectives free from 

 secondary spectrum, but these glasses are not so 

 permanent as the older types. 



(2) The series of phosphate glasses which 

 proved chemically unstable and deteriorated in 

 use. 



(3) The boro-silicate crown glasses, which are 

 of somewhat lower refractive index and dispersion 

 than the ordinary crown glass. These are good 

 glasses, and are now extensively used for small 

 objectives and for the prisms used in prism bino- 

 culars. 



(4) The dense barium crown glasses, containing 

 barium and boric acid. These glasses are used 

 in nearly all anastigmat photographic lenses, but 

 they are difficult to make because such abnormal 

 optical qualities are closely associated with chemi- 

 cal instability. 



(5) The most important result was, however, 

 the possibility of obtaining a large range of refrac- 

 tive index and dispersion, so that the designer 

 was able to regard the dispersion and refractive 

 index as more or less independent of each other. 



He could design a photographic lens with little 

 regard to the chromatic defects and correct this 

 defect by proper choice of the dispersion of his 

 glass. 



These great successes led at first to a concen- 

 tration of the optical glass industry in Jena, but 

 the success of Chance in improving the quality of 

 the older types of glass, and of Mantois in mak- 

 ing the newer types, have somewhat modified this 

 situation, though we are still dependent on Jena 

 for some of the special glasses. 



The manufacture of optical glass is not, how- 

 ever, to be lightly undertaken ; there would be no 

 great difficulty in finding the composition of 

 glasses of the existing types, though it must be 

 remembered that the mixture used will differ 

 somewhat in composition from the final product. 

 Great difficulties are associated with the purity of 

 the materials and their proper mixing; the pots 

 must be of clay, free from impurities which might 

 colour the glass. The preparation of these pots 

 requires skilled workmen of long experience, and 

 the same may be said of the melting temperatures, 

 the proper period of stirring, the rate of cooling, 

 and the whole annealing process. At the same 

 time, small variations of composition or treatment 

 will affect the optical properties quite consider- 

 ably. The experimental work associated with the 

 production of special types of glass is expensive 

 and troublesome, since the principal difficulties 

 arise when we endeavour to change from the 

 NO. 2344, VOL. 94J 



laboratory to the works scale, and at this stage a 

 single failure may cost fifty pounds. 



In view, however, of the importance to our 

 industries and to the army and navy of an ade- 

 quate supply of optical glass of various types, it 

 is most desirable that our optical glass should be 

 made in this country, and the manufacturers 

 should be encouraged in every way to meet this 

 demand. Success in this- direction is, however, 

 most likely to be achieved by scientific experi- 

 mental work carried out in conjunction with those 

 manufacturers who have already acquired valu- 

 able and essential experience in the manufacture 

 of optical glass. S. D. Chalmers. 



'THE BRITISH ASSOCIATION IN SOUTH 

 AUSTRALIA. 



Adelaide, August 12. 



THE Australian meeting of the British Associa- 

 tion opened in Adelaide on Saturday, August 

 8. The main party of overseas members arrived 

 by the Orient mail steamer Orvieto, and num- 

 bered 150. It included the majority of the 

 " advance party " to Western Australia. A smaller 

 party of fifty members had arrived by the Blue 

 Funnel steamer Euripides on the previous evening, 

 and members arriving by other and earlier routes 

 raised the total to approximately 290 overseas 

 members. The local membership roll fell but a 

 single unit short of 600 members, a figure 

 significant of the keen interest taken by the 

 people of South Australia in the advent of the 

 Association. 



The Adelaide meeting has been favoured with 

 perfect weather, and only the outbreak of the war 

 cast its gloom over a meeting which otherwise fell 

 nothing short of complete success. 



On the afternoon of arrival a special congrega- 

 tion was held by the University of Adelaide, when 

 the following honorary degrees were conferred : — 

 D.Sc.-Prof. W. J. Sollas, Prof. A. Penck, Prof. 

 T. W. Edgeworth David, Prof E. W. Brown, 

 Sir Oliver Lodge, Prof. H. Jungersen, Prof. 

 G. W. O. Howe, Dr. C. F. Juritz, and Prof, von 

 Luschan. M.D.— Prof. G. Elliot Smith. M.A.-~ 

 Mr. A. D. Hall, Prof. A. P. Coleman. B.A.— 

 Sir Charles P. Lucas. B.Sc— Prof. T. Hudson 

 Beare. 



In the same evening a reception was given to 

 the overseas members by the State Government, 

 when the visitors were welcomed by his Excel- 

 lency Sir Henry Galway, Governor of South 

 Australia, and by Mr. Peake, its Premier. 



Owing to the short space allotted to the Adelaide 

 Session, sectional meetings were not included in 

 its programme. Presidential addresses were, how- 

 ever, delivered in geography by Sir Charles Lucas 

 and in agriculture by Mr. A. D. Hall. 



Two evening discourses were also given in the 

 Adelaide Town Hall by Sir Oliver Lodge, on 

 "The Ether of Space," and by Prof. J. W. Sollas 

 on "Ancient Hunters." Both were listened to 

 with keen appreciation by a large audience. 



