NA TURE 



17, 



THURSDAY, DECEMBER 20, 1900. 



A MODERN SCIENTIFIC INDUSTRY. 

 Jena Glass and its Applications to Science and Art. 

 By Dr. H. Hovestadt. Pp. xii + 429. (Jena : Fischer, 

 1900.) 



THIS is a volume of some four hundred pages, in 

 which Dr. Hovestadt has collected a mass of 

 information about the Jena glass. 



In a report on the scientific apparatus of the London 

 Exhibition of 1876, Abbe called attention to the need for 

 progress in the art of glass making if the microscope 

 were to advance, and to the necessity for obtaining 

 glasses having a different relation between dispersion and 

 mean refractive index than that found in the material 

 then at the disposal of opticians. 



He referred to the attempts made in England by 

 Harcourt and Stokes with this object, and to the causes 

 of their failure. 



The task thus indicated was undertaken in 1881 by 

 Abbe himself and Schott at Jena. The first catalogue of 

 the Jena Laboratory, published in 1886, contains these 

 words : " The industrial undertaking which is here 

 announced for the first time arose out of a scientific 

 investigation into the connection between the optical 

 properties of amorphous fluxes and their chemical con- 

 stitution." 



The experimental work was only rendered possible by 

 repeated and large subventions from the State. The 

 immediate consequence of the undertaking was that by 

 1888 nearly all the glass required for optical work in 

 Germany was of home manufacture ; in a few years more 

 an export trade in the raw glass began, the value of 

 which in i8y8 was over 30,000/., while the value of the 

 optical instruments, such as telescopes, spectacles, field 

 glasses and the like, exported in the same year was 

 nearly 250,000/. The trade at present employs some 

 5000 workmen. 



When Abbe and Schott began their work, some six 

 elements only entered into the composition of glasses. 

 By 1888 it had been found possible to combine with these 

 six quantities, up to at least 10 per cent., of twenty- 

 eight additional elements, and the effect of each of 

 these on the refractive index and dispersion had been 

 determined. 



Thus, for example, these investigators had found that 

 by the addition of boron the ratio of the length of the 

 blue end of the spectrum to that of the red is reduced ; 

 while fluorine, potassium and sodium produce opposite 

 results. , 



Now an ordinary achromatic lens, uniting two colours 

 of the spectrum, is formed by combining a crown glass 

 lens with one of flint glass having equal total disper- 

 sion ; but though the total dispersion is the same for the 

 two it is differently distributed throughout the spectrum. 

 In the flint glass the dispersion of the blue end is greater, 

 that of the red less, than in the crown ; hence the light 

 from a white source is not white after traversing the 

 lens ; a " secondary " spectrum remains, and it was the 

 existence of this which rendered the progress of the 

 microscope so difficult. Abbe's experiments showed how 

 NO. 1625. VOL. 63] 



the difficulty was to be met. By combining a high pro- 

 portion of boron with flint glass, its spectrum became 

 more nearly the same as that of a crown glass. Such a 

 glass had been made by Harcourt many years previously, 

 while a glass containing phosphates instead of silicates 

 is found to have the same dispersion as, combined with 

 a higher refractive index than, the ordinary crown 

 glasses, and therefore serves better to achromatise the 

 borate-flint glass. In fact. Abbe showed that with two 

 such glasses it is possible to combine three colours 

 instead of only two ; the outstanding spectrum is greatly 

 reduced in length, and is called a "tertiary" instead of a 

 "secondary" spectrum. 



Again, the ordinary microscope lens of two glasses can 

 be corrected for axial spherical aberration for one colour 

 only. Abbe showed that the new borate-phosphate 

 lenses could, by combination with a lens of fluor-spar, 

 have their axial spherical aberration corrected for two 

 colours. These lenses he called apochromatic. 



It was found more difficult to reduce the secondary 

 spectrum by lengthening the red end of the spectrum of 

 the crown glass. This required the addition, as we 

 have said, of fluorine, potassium or sodium. The effect 

 of sodium is small ; glasses with a large amount of potass- 

 ium can be made, but are very hygroscopic, while the 

 introduction of fluorine though it was successfully effected^ 

 is involved with many difficulties. 



The book under review gives, in its first two chapters, 

 an account of the preliminary work of Abbe and Schott, 

 and full details as to the optical properties of the glasses 

 now made. The next four chapters deal with the optical 

 instruments manufactured out of the glasses. 



We have already referred to the fundamental improve- 

 ment in the microscope rendered possible by their use ; 

 the problem to be solved in the case of a photograph 

 lens was somewhat different. It follows, from the work 

 of von Seidel, that, with the ordinary crown and flint 

 glasses, the conditions for achromatism and for flatness 

 of field cannot be satisfied together. To do this it is 

 necessary to find a glass of high refractive index and 

 low dispersive power, or vice versd. In ordinary glasses 

 refractive index and dispersive power go together. 



Thus, ordinary hard crown glass has a refractive index 

 of i'5i8 and a dispersive power of •0166, while for extra 

 dense flint the figures are 1717 and '0339. An achro 

 matic lens might be constructed out of these two glasses, 

 but the field could not be flat. 



By introducing barium, however, into the crown glass, 

 a change is produced in this respect. Thus for barium 

 silicate crown the refractive index and dispersive power 

 are 1*573 and "0173, while for soft crown they are i '5 15 and 

 •0177. With these two glasses, the problem of construct- 

 ing a photographic object-glass possessing achromatism 

 and flatness of field becomes possible. For the various 

 methods of solution we must refer to the book ^ itself, in 

 which also will be found details as to the use of the glasses 

 for telescopic lenses. 



The mechanical properties of glass are next considered, 

 and in Chapter ix. we come to a careful discussion of 

 the imperfect elasticity of glass, specially in connection 

 with thermometry. 



1 See also " Contributions to Photographic Optics," by Dr. Otto Lummer, 

 translated by Prof. S. P. Thompson. 



