20 Transactions of the Society. 



III. — 0)1 Improvements of the Microscope with the aid of New Kinds 

 of Optical Glass* 



By Prof. E. Abbe, Hon. F.E.M.S. 



(Read ISth October, 1886.) 



Since the year 1881, Dr. Schott, of Jena, and the author, with active 

 co-operation from the optical workshops of Zeiss, have undertaken 

 a prolonged investigation into the improvement of optical glass, the 

 result of which has been the production of new kinds of glass for the 

 use of opticians. 



By spectrometric observation of numerous experimental fusions, 

 systematically carried out with a great variety of chemical elements, the 

 relation between the optical properties of the amorphous (glassy) pro- 

 ducts and their chemical composition has been more closely investi- 

 gated; and on the basis of the results so obtained suitable syntheses 

 have been made by which it has been possible to produce glass having 

 desired optical properties. 



In this way, by the use of many more chemical elements than have 

 hitherto been employed in the manufacture of glass, especially by the 

 use of phosjDhoric and boric acid as essential constituents of glass fluxes, 

 where formerly silica was alone used, two hitherto unattainable require- 

 ments of practical optics have been satisfied. In the first place, crown 

 and flint glass can be produced in which the dispersion in the difi'erent 

 parts of the spectrum is nearly proportional, so that in achromatic com- 

 binations it is now possible entirely, or almost entirely, to do away 

 with the hitherto unavoidable secondary spectrum ; secondly, the kinds 

 of glass which can be used for optical purposes have been so increased 

 in variety that, while the mean index of refraction is constant, con- 

 siderable variations can be given to the dispersion, or to the refractive 

 index while the dispersion remains constant ; in particular, a high index 

 of refraction is no longer necessarily accompanied by a high dispersion 

 (in flint glass), but may be retained (in crown glass) with a low degree 

 of dispersion. 



The regular supply of optical glass in answer to such increased 

 demands seems to be insured for the future, since as a direct result of 

 the above-mentioned experiments, and with the co-operation of the Koyal 

 Prussian Education Office, a glass factory has been established at this 

 place (the Glastechnisches Laboratorium of Jena), which has meanwhile 

 commenced the manufacture of all kinds of optical glass for general use. 



The circular which has recently been issued by this institution gives 

 preliminary information of a more detailed character to those who are 

 interested in the subject, prior to the appearance of a complete description 

 of the results of the experiments. 



The new materials which are thus placed at the disposal of practical 

 optics by an extension of glass manufacture from a scientific point 



* The original paper is written in German (translated by Mr. H. A. Miers of the 

 British Museum, Natural History, and revised by the author). Cf. SB. Jen. Gesell. f. 

 Med. u. Naturw., 9th July, 1886, 24 pp. 



