Ch. XII] fflSTORY OF LENSES AND MICROSCOPES 437 



him to produce the objectives of his dream with large aperture and 

 perfect color and spherical correction. While his success, and that of 

 his pupil ToUes, were certainly great in producing the highest type of 

 objective for the telescope and microscope with the materials aheady 

 to be had, his glass making did not bring him all that he wanted. It 

 was reserved for the optical works of Zeiss and the genius of Abbe, 

 with the hfelp of the practical glass maker Schott, and the liberality 

 of the German government to finally overcome the difficulties in mak- 

 ing new forms of glass with specially desired quahties of dispersion 

 and refraction; and even then it was necessary to go back to the 

 natural mineral fluorite to make possible the apochromatic objec- 

 tives. Those interested are recommended to read the work of Hove- 

 stadt on the new Jena glass. 



§ 702. Immersion objectives. — In the development of any art 

 the science needed almost always lags behind, and is developed in 

 most cases to explain what has already been discovered by the hard 

 and roimdabout method of " trial and error." This was the case 

 with immersion objectives. Amici in Italy and David Brewster in 

 Great Britain were busy in tr5dng to improve microscope objectives 

 by any feasible method. They used aU sorts of Hquids for immersion. 

 Water was one of the most successful and still holds its own. 



§ 703. Homogeneous immersion objectives. — The advantage of 

 the immersion principle gradually became understood to be the 

 possibUity of increasing the aperture under which the object could be 

 viewed. The final step by which the aperture could be pushed to the 

 limit of human skill in figuring the lenses came when Mr. Tolles (1871- 

 1874) showed in the clearest manner the possibihty of making such 

 objectives and increasing the aperture by means of homogeneous 

 contact between the condenser and the slide or object and between 

 the object or cover-glass and the front lens of the objective. The mat- 

 ter is weU stated by Hon. J. D. Cox in his presidential address before 

 the American Microscopical Society for 1884 (pp. 5-39), and in Mr. 

 Mayall's Cantor Lectures on the History of the Microscope (1885). 

 On p. 96 MayaU says: " If priority of pubUcation of the formula on 

 which homogeneous immersion objectives could be produced carries 

 with it the title of inventor, then Mr. R. B. Tolles stands alone as 



