June i, 1905] 



NA TURE 



^15 



of the pencil through the object glass. Abbe soon realised 

 the defects of the ordinary theory. He found it necessary 

 to apply the principles of the wave theory, the teaching of 

 Voung and Fresnel, to the problem, and was led in 1870 

 to the theory of microscopic vision which bears his name. 

 His worli was the direct outcome of that of Fresnel. 



He soon realised that it followed from the mathematical 

 theory that with the glass then at the optician's disposal 

 no great improvement in the microscope object glass could 

 be expected. Certain relations between the dispersion and 

 refraction in the various lenses were requisite to secure 

 achromatism, and no glass having these relations existed. 

 An inspection of the instruments in our loan exhibition at 

 South Kensington in 1876 confirmed this view, and he 

 published it in a report in 1878 on the results of the ex- 

 hibition : — " The future of the Microscope as regards its 

 future improvement in its dioptric qualities seems to be 

 chiefly in the hands of the glass maker." 



The investigations of Petzval and of von .Seidel led 

 to a similar result with regard to photographic lenses. 

 Von Seidel 's work dates back to 1856-7, but his main 

 paper was not written until 1880, after the date of .Abbe's 

 report, and was not published in full until 1898. 



It follows from these investigations that with the glass 

 then on the market it was impossible to make the field 

 of a photographic lens at once fiat and achromatic. 



Thus the theoretical work indicated a bar to future 

 progress which could only be removed by the manufacture 

 of new glasses having certain definite properties. It is 

 fitting to say that at an earlier date this fact had been 

 recognised by our countrymen Mr. Vernon Harcourt and 

 Prof. Stokes, who for some eight years previous to 1870 

 had endeavoured, but with scant success, to make the 

 glass required. 



Abbe was more fortunate ; his report fell into the hands 

 of Dr. Otto Schott, a glass maker of Witten, in West- 

 phalia, who realised its importance. In 1881 Schott com- 

 municated with Abbe, and the next year he removed to 

 Jena, and the firm of Schott and Partners was born. 



In the first catalogue of the Jena Glass Works they 

 write : — " The industrial undertaking here first brought 

 into public notice and which has arisen out of a scientific 

 investigation into the dependence between the optical proper- 

 ties and the chemical composition of solid amorphous 

 fluxes was undertaken by the undersigned (Schott and 

 Abbe) in order to discover the chemicophysical foundations 

 of the behaviour of optical glass." The inquiry was aided 

 by large grants from the Prussian Minister of Education. 

 The practical result is seen in the catalogue of the Jena 

 firm and the enormous export of German optical goods. 



Nor is this all, for in virtue of the distribution of profits 

 settled by the scheme of the Carl Zeiss Stiffing, drawn 

 up by Abbe some years ago and ratified by the Bavarian 

 Government, the University of Jena alone has received 

 a sum approaching ioo,oooi. Abbe's work at Jena is 

 perhaps tlie most striking illustration of the way in which 

 progress depends on the cooperation of science and ex- 

 perience. One could give statistics to illustrate the truth 

 of this and the important effect it has had on German trade 

 and prosperity. They are hardly necessary ; the facts are 

 patent, and their cause well known to all who care to 

 inquire. We can progress too if we follow the path laid 

 down for us of old by N'ewton, Young, Herschel, .4iry, and 

 I he others of whom I have spoken. 



Exhibition of Optical and Scientific Instruments. 

 The exhibition of optical and scientific instruments 

 which is being held during the present week at the 

 Northampton Institute, Clerkenwell, E.C., in connection 

 with the optical convention, presents many features of 

 interest, and all who have had any e.xperience in the use 

 of an optical instrument, from the wearing of a pair of 

 spectacles to the handling of an accurate spectrometer, will 

 find something to repay the trouble of a visit to Clerken- 

 well, still the centre of the optical industry. While the 

 number of actual novelties offered is not, perhaps, very 

 large, there are few classes of instruments unrepresented, 

 and though the names of certain important firms are con- 

 spicuously absent from the list of exhibitors, the exhibition 

 as a whole may be taken as well representative of the 



NO. 1857, VOL. 72] 



activities of the British manufacturers of optical and other 

 scientific instruments. 



In the main of an optical character, the scope of the 

 exhibition has been extended to cover such other scien- 

 tific instruments as are usually manufactured by optical 

 instrurnent makers. Meteorological instruments and thermo- 

 meters, mathematical and drawing instruments and calcti- 

 lating machines, and laboratory apparatus generally, are 

 thus included. Electrical measuring instruments, however, 

 are not shown. It is for many reasons to be regretted that 

 the exhibition has been confined to the work of British 

 makers ; a foreign section would have had much interest 

 for the ordinary visitor, and would have been of great 

 educational value both to the British manufacturer and 

 his competitors; we understand, however, that the limit- 

 ation was dictated by considerations as to space, and the 

 necessity of restricting the magnitude of a somewhat novel 

 undertaking. 



In the catalogue which has been prepared in connection 

 with the exhibition, the convention committee is to be 

 congratulated on having produced a volume which should be 

 of considerable value as w^ell to the user of scientific instru- 

 ments as to the firms whose instruments are there de- 

 scribed. The volume is not confined to apparatus actually 

 exhibited ; the aim has been to provide a convenient work 

 of reference generally descriptive of the productions of 

 British firms, and in which particulars as to the types 

 offered by different makers of any special instrument may 

 be readily found. To this end the instruments have been 

 arranged in classes, which are in many cases further sub- 

 divided, and in addition to a table of contents, an alpha- 

 betical list of exhibitors, with general information as to 

 their manufactures, and an index of instruments have been 

 provided. k short introduction to each class furnishes 

 some particulars as to the instruments included thereunder, 

 with notes as to recent advances in the mode of construc- 

 tion. 



In class i., tools and materials, the most interesting 

 exhibit is that of Messrs. Chance Bros., which includes 

 some varieties of optical glass only quite recently produced 

 by the firm, and not previously shown. Some special opal 

 glass of low coefficient of expansion for speculum discs is 

 also exhibited. Messrs. Jas. Powell and Sons, of the 

 Whitefriars Glass Works, show specimens of glass for 

 thermometers and other purposes. Tools for lens grind- 

 ing, and exhibits illustrating processes of manufacture, are 

 shown by Messrs. Geo. Culver and other firms. 



Class ii., simple elements and instruments, includes some 

 accurate glass work by Messrs. A. Hilger, while Lord 

 Blythswood shows specimens of his diffraction gratings 

 ruled on speculum metal, 14,400 lines to the inch, up to 

 a length of 6 inches. Replicas of Rowland gratings, with 

 spectroscopes of various forms in which they are employed, 

 are shown by Mr. T. Thorp, of Manchester. 



Class iii., astronomical instruments, and class iv., 

 nautical instruments, are by no means representative of the 

 best English work, and it is to be regretted that the cata- 

 logue is here so meagre. 



In class v., surveying instruments, on the other hand, 

 few firms of importance are omitted, and some excellent 

 work is shown. In particular may be mentioned the 

 Wells theodolite of Messrs. Elliott Bros., which embodies 

 several novel features ; Messrs. Joseph Casartelli and Son, 

 of Manchester, also show instruments of somewhat 

 special pattern. Messrs. W. F. Stanley, J. J. Hicks, and 

 E. R. Watts and Son are well represented. The chief 

 characteristics of the more modern instruments are the use 

 of larger and more powerful telescopes, and the increased 

 accuracy of graduation. 



Class vi. is devoted to range finders and heliographs, and 

 the exhibits of most interest are the naval and field range 

 finders of Messrs. Barr and Stroud, and the stereoscopic 

 range finder of Prof. Geo. Forbes. Messrs. Ross, Ltd., 

 show specimens of their new variable power gun sighting 

 telescopes, in which by a simple device the power can be 

 altered while the image remains always in focus on the 

 cross wires. 



Class vii. includes meteorological instruments and thermo- 

 meters, and most of the well known makers have sent 

 exhibits. In class viii., spectacles and eyeglasses, the ex- 

 hibits are also sufficiently representative of the best English 



