318 SUMMARY OP OUEEENT BESE ARCHES RELATING TO 



red rays and over-coirectei for the blue rays. In the new objectives, 

 however, the correction of the spherical aberration is obtained for 

 two different rays of the spectrum, that is practically for all colours at 

 the same time, and the objective shows the same degree of chromatic 

 correction for the central as for the marginal part of the aperture. 

 All this requires greater complication in the construction, hence the 

 use of five lenses instead of the four hitherto employed. In addition, 

 uniformity of amplification by the various zones of the clear aperture 

 has been obtained in a higher degree than could hitherto be done. 



The objectives will be specially useful in photo-micrography where 

 the correction of the secondary spectrum will be found of considerable 

 practical advantage. Not only is there no difference in the optical 

 and chemical foci, but the image formed by the chemical rays is in 

 itself much more perfect. This advantage is very clearly verified by 

 experimental trials which have been made. For photo-micrography a 

 third eye-piece magnifying 2^ times is supplied, the lenses of which 

 can be sKghtly separated for exact adjustment of the image. 



Two series of objectives will be constructed, one adapted for the 

 short Continental body-tube and the other for the long English body- 

 tube, and there will be a corresponding "compensating" series of 

 eye-pieces. The homogeneous-immersion lenses will have aper- 

 tures of 1-40 N.A. and 1'30 N.A., and focal lengths of 3'0 mm. 

 and 2 • mm., the latter with much increased working distance. The 

 water-immersion lenses will have an aperture of 1 • 25 N.A. and a 

 focal length of 2' 5 mm., and the dry lenses 0*95 N.A., 0-60 N.A., 

 and 0*30 N.A., with focal lengths of 4 mm,, 8 mm., and 16 mm. 



We append what will we think be of interest to many of the 

 Fellows, a brief account of what we understand to be the history of 

 the construction of the new glass, though, as we have not been able 

 to submit it to Prof. Abbe, he must not be understood to endorse it 

 in any way. 



The origin of the matter was Prof. Abbe's Eeport on the Micro- 

 scopes of the South Kensington Exhibition published in 1878.* This 

 contained at the end some general considerations as to the unfulfilled 

 requirements of practical optics in regard to the properties of optical 

 glass, and complaints of the unfavourable conditions then existing. 

 Dr. 0. Schott (of Witten, in Westphalia), a chemist, but long versed 

 in practical glass-making, and who had made some remarkable re- 

 searches on the physical properties of glass, read the report, and in 

 the beginning of 1881, having communicated with Prof. Abbe, they 

 commenced a preliminary study of the optical properties of the various 

 chemical elements as far as they admit of " vitrificable " combinations. 

 This was conducted at first on a very small scale, Dr. Schott working 

 alone at Witten, and the optical part of the research being carried out 

 at Jena. After a year it was decided to continue the experiments on 

 a larger scale, with the object not only to determine the optical effects 

 of various elements, but to try the production of practically useful 

 combinations. In January 1882, Dr. Schott settled at Jena, and he 

 and Prof. Abbe established a complete melting-laboratory with large 



* See this Journal, iv. (1884) p. 291. 



