236 



-NATURE 



[May 22, 19 19 



an interesting account of the design of electric reduc- 

 tion furnaces for the production of pig-iron from ore, 

 a process which becomes economically sound when 

 I horse-power-year of electrical energy does not cost 

 more than 2-3 tons of coke, and is now coming 

 into extensive use in Sweden and elsewhere. In this 

 country^ steel refining for ingots and castings and the 

 production of ferro-manganese and steel alloys con- 

 stitute the chief uses of electric furnaces at present. 



The relative merits of amorphous carbon and 

 graphite for electrodes were dealt with in several of 

 the papers read, the balance of evidence being in 

 favour of graphite, which, owing to its superior con- 

 ductivity, permits of the use of narrower electrodes. 

 Dolomite is generally used to form the hearth, but 

 acid linings are said also to be employed in some 

 cases. In spite of the higher cost of heat produced 

 electrically over the use of fuel, the superior quality 

 of the products, the small wastage by oxidation, and 

 the ease with which scrap may be utilised justifv the 

 use of the electric furnace. It is to be hoped that the 

 experience trained with steel will lead to the produc- 

 tion of artificial abrasives such as carborundum and 

 alundum in Britain, and also to the development of 

 the higher refractories needed in many metallurgical 

 processes. Chas. R. Darling. 



BRITISH OPTICAL RESEARCH. 



\17"E have before us several books and a large 

 ** number of reprints from various scientific 

 publications, all of which represent work done by 

 members of the scientific staff of Messrs. Adam 

 Hilger, Ltd., since the beginning of the war. We 

 must welcome not only the fact that a British optical 

 firm has realised the value of a considerable staff of 

 highly qualified scientific collaborators, but more par- 

 ticularly the circumstance that this staff is encouraged 

 by the firm in the publication of its work, and' in 

 thus helping to hasten the recovery by this country 

 of the leading position in applied optics which it 

 undoubtedly held in a rather distant past, but which 

 it had almost com,pletely lost in more recent years, 

 largely through the narrow outlook of a majority of 

 optical firms! in seeking only imm.ediate and certain 

 profit and keeping down or totally excluding "non- 

 productive " labour, but also through the failure of 

 our educational institutions to teach real optics 

 capable of application to actual technical problems 

 instead of the transparent sham beloved by examiners 

 and their text-books. 



From the practical optician's point of view the 

 most valuable of the publications are probably those 

 by Mr. Twyman, the present head of the firm, which 

 deal with the Hilger interferorneter for the correction 

 of lenses and prisms {Phil. Mag., January, 1918, and 

 Photogr. Journ., November, 1918). By directly in- 

 dicating the residual imperfections of a lens or prism 

 in the form of a contour-map built up of interference- 

 fringes, this instrument enables a skilled workman 

 systematically to remove those imperfections and to 

 perform, without other guidance, the process of "figur- 

 ing" which hitherto had to be directed by a highly 

 skilled and experienced observer on the basis of 

 repeated tests of the lens or prism by the in- and 

 out-of-focus appearance of a real or artificial star, 

 and which then was an expensive, slow, and uncertain 

 operation. For the present this valuable method is, 

 unfortunately, limited to small sizes owing to the cost 

 and difficulty of producing large piano-parallel plates 

 of the requisite almost absolute perfection. 



Mr. Twyman also contributes an instructive paper 

 on the annealing of glass (Trans. Soc. of Glass 

 Technol., vol. i., 1917), which deals more especially 

 NO. 2586, VOL. 103] 



with the importance of passing the glass very slowly 

 through a comparatively short range of temperature. 

 In describing methods of fixing this range, and in 

 working out the law according to which the viscosity 

 of the glass increases within the critical range, Mr. 

 Twyman goes decidedly beyond the publications of 

 the Jena works on this subject of "fine annealii^g." 



Two members of the staff, Mr. R. G. Parker and 

 Mr. A. J. Dalladay, describe another valuable innova- 

 tion in optical precision work, viz. the permanent 

 union of very closely fitting polished glass surfaces by 

 raising them to a very closely gauged temperature 

 at which they become welded together without any 

 distortion which would affect their optical perfection 

 (Trans. Faraday Society, vol. xii., part 1,1916). In 

 the case of glasses which agree sufficiently closely 

 in their rate of expansion, this promises to prove a 

 very decided improvement on the usual cementing 

 processes. 



In an interesting paper to the Physical Society 

 (Proc, vol. XXX., part iii.) Mr. Simeon discusses the 

 accuracy attainable with critical angle refractometers. 

 As is probably widely known, these instruments are 

 now built bv Messrs. Hilger, Ltd. 



Dr. L. Silberstein, the scientific adviser of the 

 firm, is widely known as an extremely able mathe- 

 matical physicist. His two books on "The Electro- 

 magnetic Theory of Light" and on "A Simplified 

 Method of Tracing Rays" have already been reviewed 

 in these columns. In the collected researches before 

 'us we find five additional contributions from his pen 

 to the Phil. Mag. A paper on " Fluorescent Vapours 

 and their Magneto-optic Properties " and two on 

 "Molecular Re'fractivity and Atomic Interaction " are 

 purely theoretical investigations on subjects only 

 remotely connected with technical optics. In a paper 

 on "Multiple Reflections" (November, 1916) Dr. 

 Silberstein gives a very general treatment, by his 

 favourite vectorial method, of the reflection of light 

 at combinations of plane mirrors, more particularly 

 with the view of elucidating the behaviour of the 

 important "central" or "corner-cube" mirrors which 

 have proved so valuable for signalling and range- 

 finding purposes. Finallv, there is a paper on "Light 

 Distribution round the iFocus of a Lens at Various 

 Apertures " (January, 1918), in which the problem of 

 the spurious disc in the presence of spherical aberra- 

 tion is attacked.. One would like to see the subject 

 worked out in a more practically useful form ; the 

 example of the phenomena at the paraxial focus of a 

 plano-convex lens which is chosen for numerical treat- 

 ment is not very interesting, and there is an obvious 

 numerical error "in the working out of the "best rela- 

 tive aperture" on p. 47. By the author's own formula 

 (20) this comes out at rather more than twice the 

 stated values, and the results then agree fairly well 

 with everyday experience as to the permissible aper- 

 ture of plano-convex lenses as used in ordinary eye- 

 pieces and magnifiers. But, apart from this little 

 slip, the matter of real interest to optical designers is 

 the appearance of the ima^e at the point of best con- 

 centration of the light, which is easily shown to lie 

 very nearlv midway between the geometrical foci of 

 the paraxial and marginal ravs respectively, for in 

 this position the maximum difference of phase is only 

 one-fourth of that at either the paraxial or the mar- 

 ginal focus. At the correctly worked out "best rela- 

 tive aperature" the lens of !^-i,6 cm. radius of curva- 

 ture chieflv calculated for bv Dr. Silberstein has a 

 longitudinal spherical aberration of about o-6 mm., 

 and the interesting region would therefore be found 

 about 0-3 mm., sav 600 A. from the paraxial focus. 

 It is, therefore, not surorising that the author finds 

 no appreciable change in the light distribution on 

 trving a change of focus of " even " 10 A. It is greatly 



