ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 555 



stands almost alone. It is a scientific discourse upon an entirely new 

 series of optical metals. It would be well if we had accessible anywhere 

 an equally accurate and efficient account of optical glasses in use before 

 this remarkable and most valuable series of Jena glasses were devised 

 and made accessible. 



As a treatise it is a monument to the scientific knowledge, skill, 

 ingenuity, and indomitable resolution of German men of science. The 

 labour must have been great ; the book is practically a record of various 

 experiments which have been made to discover the composition needful 

 to obtain a series of optical fluxes which should possess the properties 

 optical and mechanical for securing results that had been before optically 

 impossible. Sir I. Newton had satisfied himself that the hindrance to 

 the production of a perfect optical instrument, such as a telescope, was 

 not the production of perfect figures in the glasses, but the different 

 refrangibility of the rays of light. In the glasses used in the construc- 

 tion of optical instruments prior to the production of the optical fluxes 

 of Jena, two kinds of glass having proportional dispersion powers could 

 not be found ; as is well known, " irrationality of spectrum " resulted 

 and absolute chromatic correction could not be accomplished. The 

 want of proportion in the dispersion of the various colours of the 

 spectrum in two kinds of glass, such as were obtainable before the Jena 

 glasses were produced, left a colour or colours outstanding in " corrected " 

 or achromatic combinations of, for example, microscopic object-glasses, 

 known as the secondary sjjectrnm. 



It is by the production of the most ingenious vitreous compounds of 

 which this book gives careful history and elaborate scientific details, 

 combined with fluor-spar, that this secondary spectrum was removed 

 and a new era for microscopic objectives and work inaugurated ; and in 

 every field in which optical instruments are used an immensely im- 

 portant series of improvements have resulted. 



Amici showed that the introduction of a drop of water between the 

 first surface of the object-glass and the covering glass of the object 

 would diminish the loss of light which arose from the passage of the 

 rays from the object into air before reaching the objective. Sir David 

 Brewster had seen and suggested this as far back as 1813, and its 

 adoption was known as the " water-immersion." Clearly, however, when 

 the rays enter the object-glass from water instead of air, both its 

 refractive and dispersive action will be altered ; and important con- 

 structive modification would be needed to suit the new conditions. 

 Hartnack was the first to successfully bring this about, and the immer- 

 sion system was introduced. This system was still more powerfully to 

 influence the future of the Microscope, under the now famous homogene- 

 ous system of immersion. This system was first suggestively employed 

 by Tolles ; but Prof. Abbe had at the same time a more or less clear 

 perception of its potential value. " The matter assumed, however, sub- 

 sequently, a different shape in consequence of a suggestion made by 

 Mr. John "Ware Stephenson ... of London, who independently dis- 

 covered the principle of homogeneous immersion." * 



* Abbe, tbis Journal, ii. (1879) p. 257. 



