90 



THE AMEKICAN MONTHLY 



[May, 



and chemical foci, but the image 

 formed by the chemical rays is in 

 itself much more perfect. This ad- 

 vantage is very clearly verified by 

 experimental trials which have been 

 made. For photo-micrographv a 

 third eye-piece magnifying 2^ times 

 is supplied, the lenses of which can 

 be slightly separated for exact ad- 

 justment 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 ^ye-pieces. 

 The homogeneous-immersion lenses 

 will have apertures of i .40 n.a. and 

 1 .30 N. A. , and focal lengths of 3 .0 mm . 

 and 2.0 mm., the latter with much in- 

 creased workingdistance. 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 under- 

 stand to be the history of the con- 

 struction of the new glass, though, as 

 we have not been able to submit it 

 to Prof. Abbe, he must not be under- 

 stood to endorse it in any wav. 



' The origin of the matter was 

 Pi'of. Abbe's Report on the Micro- 

 scopes of the South Kensington Ex- 

 hibition j^i^iblished in 1878.* This 

 contained at the end some general 

 considerations as to the unfLiIfilled 

 requirements of practical optics in 

 regard to the properties of optical 

 glass, and complaints of the unfavor- 

 able conditions then existing. Dr. 

 O. Schott (of Witten, in Westphalia) , 

 a chemist, but long versed in practi- 

 cal glass-making, and who had made 

 some remarkable researches on the 

 physical properties of glass, read the 

 report, and in the beginning of iSSi, 

 having communicated with Prof. 



*Journ. R. Micr. Soc, iv, (1884) 291. 



Abbe, they commenced a preliminary 

 study of the optical properties of 

 the various chemical elements as far 

 as tliey admit of " vitrificable " com- 

 binations. This was conducted at 

 first on a ver}^ small scale. Dr. Schott 

 working alone at Witten, and the op- 

 tical part of the research being car- 

 ried out at Jena. After a year it 

 was decided to continue the experi- 

 ments on a larger scale, with the 

 object not only to determine the op- 

 tical 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 com- 

 plete melting-laboratory with large 

 gas-furnaces, a gas engine for driving 

 blowers, &c., and with the aid of 

 two assistants for the chemical and the 

 optical part of the work, and of sev- 

 eral workmen, tiie experimental re- 

 search was continued there for two 

 years. 



*■ The general direction of the work 

 was based on the principles indicated 

 in the Report of 187S, and in the 

 paper in this Journal before men- 

 tioned. According to these princi- 

 ples, there were two distinct ob- 

 jects : — (i) To obtain a greater vari- 

 ety of the optical properties of the 

 glass in regard to the relation of the 

 refractive to the disjoersive power. 

 The existing kinds of optical glass 

 constituted nearly a line, i. e., the 

 dispersion increasing always with 

 the refraction, with very slight devia- 

 tions only. The object was to com- 

 bine glasses which, if arranged ac- 

 cording to n and /\ n, wovdd not be 

 confined to a linear series, but would 

 embrace an area of a certain breadth, 

 one value of n admitting various 

 values of /\ n, and vice versa, as far 

 as possible. 



' (2) The second problem was : — 

 To procure kinds of glass of dift'er- 

 ent relative dispersions, in which 

 the dispersions should be propor- 

 tional, as near as possible, in differ- 

 ent parts of the spectrum (the prob- 

 lem of '•'secondary chromatism ") . 



