364 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



prolonged use of the oxy-hydrogen gas furnace, and the satisfactory 

 results from the testing of a series of prisms. They give tables of : 



1. The refractive indices of vitreous silica. 



2. The focal lengths in metres of a compound lens of fluorite and 

 vitreous silica, achromatised for wave-lengths 7050 and 1852. 



3. Partial and proportional dispersions of fluorite and vitreous silica. 

 The second of these is quoted below in extenso. 



Radii :— R = 0-38733; S = 0-20351 ; R' = S ; S' = ao. 



R, S, R', S' refer to the fciirfaces of the two lenses. 



Theories of the Resolving Power of a Microscope.* — Geometrical 

 optics in its relation to instruments has, says R. T. GT.,f been studied to 

 great advantage abroad ; we in England have of recent years somewhat 

 neglected the subject, with the result that only a small share in the 

 recent advance in lens construction has been ours. The books and 

 papers under review tell us of the advance. 



It was in 1878, in his report on the London International Exhibition 

 of Scientific Apparatus, that Prof. Abbe first directed attention to the 

 fact that the further perfection of the Microscope as an optical instru- 

 ment depended on the advance of the art of glass making. With the 

 glasses then at their disposal it was not possible for opticians to get rid 

 of the secondary spectrum of their object glasses ; while a glass could 

 be made achromatic for two wave-lengths, the differences in the relative 

 dispersion of the two ends of the spectrum were such that there was an 

 outstanding amount of colour which prevented the attainment of the 

 highest perfection of the image. It was to this fact that the establish- 

 ment of the now celebrated firm of Schott and Company was due, and 

 the results of Abbe's own work on Microscope lenses are summed up in 

 the first volume of his collected papers, which has recently appeared. 



' Gesarumelte Abhandlungen.' Von Ernst Abbe. 



' Das Zeisswerk und die Karl Zeiss-Stiftung in Jena.' 



' Zur Tbeorie der Mikroskopiscken Bild-erzeugung.' By Victor Grunberg. 



'The Helmholtz Theory of the Microscope.' By J. W. Gordon. 



' The Theory of Optical Images.' By Lord Rayleigh (Joiiru. R.M.S., 1903). 



Nature, Ixix. (1904) pp. 497-8. 



