32 ELEMENTARY PRINCIPLES OF MICROSCOPICAL OPTICS 



fractive index could be united with a. higher dispersive power, or a 

 higher refractive index with a relatively lower dispersive power. 

 By proper combination of such materials, if they be provided with 

 ordinary crown and flint glass to partly remove the chromatic and 

 spherical aberrations independently of each other, and so to obey 

 the conditions on which the removal of the chromatic difference 

 depends, these aberrations could be compensated. 



All this was seen and fully demonstrated and set forth by Abbe 

 as far back as 1876, 1 and he pointed out that the further perfecting 

 of the microscope in its dioptrical working w r as dependent on the 

 art of glass making ; the production, that is to say, of vitreous 

 compounds possessing different relations of refractive and disper- 

 sive power by [means of which the secondary spectrum could be 

 removed. 



For practical purposes the matter was in abeyance until 1881, 

 but since that time Dr. Schott and Professor Abbe, with the active 

 co-operation of the optical workshops of Zeiss, undertook the 

 laborious and prolonged investigation into the improvement of 

 optical glass, to which we have alluded ; the result has been the 

 production of ' crown ' and * flint ' glass possessing exactly the 

 qualities foreshown as indispensable by Abbe. 



By chemical, physical, and optical research of a most laborious 

 nature, and by spectrometric observations of numerous experimental 

 fusions systematically carried out with a large variety of chemical 

 elements, the relation between the vitreous products and their 

 chemical composition has been more closely investigated. 



In the crown and flint glass produced up to the time of these 

 investigations, the uniformity of property arose from the relatively 

 small number of materials employed. Aluminium and thallium, 

 with silica, alkali, lime, and lead, formed the limit. By the use of 

 more chemical elements, especially phosphoric and boric acid as the 

 essential constituents of glass fluxes in the place of silica alone, flint 

 and crown glass have been produced in which the dispersion in the 

 different parts of the spectrum is nearly proportional ; so that in 

 achromatic combinations it is now a question of detail and practical 

 optics to eliminate almost entirely the secondary spectrum. It is 

 unfortunate, nevertheless, that a large number of these glasses, 

 especially those of most value to the optician, have proved to be so 

 unstable in their composition that opticians refrain from using them. 

 It may be hoped that further experiment and research will greatly 

 reduce this defect. On the other hand, 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, considerable 

 variations can be given to the dispersion or to the refractive index 

 while the dispersion remains constant. A high index of refraction 

 is no longer of necessity accompanied by a high dispersion in 

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

 of dispersion. 



The practical consequence of this is that both the imperfections 



1 Hoffman, A. W., Bericlit uber die wissensclia ft lichen Apparate anf der Lon- 

 doner Internationalen AussteUnng im Jalire 1876. 



