18 Transactions of the Society. 



superiority over those of his contemporaries, but eventually he adopted 

 the Lister formula and continued to make use of it for upwards of 

 fifty years. 



There can be no doubt that up to the time of the Great Exhibi- 

 tion of 1851 * the objectives made by Ross, Smith, and Powell, 

 on the Lister formula, were far superior to those made on the Con- 

 tinent or anywhere else ; they had greater aperture, were more 

 achromatic, and had less spherical aberration than any other lenses- 

 of that date. This is not a matter of theory or of conjecture but 

 of lact, for the lenses are in existence, and may be tested by any one 

 wishing to satisfy himself on this point. After this the Amici 

 water-immersion, the solid front, and other devices, in which Mr. 

 Lister took no part, were introduced ; these greatly improved the 

 higher powers, but the lower powers constructed on Lister's principles 

 remained without a rival for fifty years ; then at last his £ had to give 

 way to one with a duplex front, but the other powers, viz. the T %, ^, 

 §, 1-in., li, 2-in. still remained the best until the advent of the 

 apochromatics and semi-apochromatics in 1886. These facts are 

 sufficient to demonstrate the value and importance of Lister's work. 



* The following extracts are from the Jurors' Report, Great Exhibition, 1851 : — 

 "Nachet (France). The object-glasses, though inferior to both those of Boss 

 and Smith and Beck, are by far the best of the foreign ones." 



"Chevalier (France). A Microscope with indifferent object-glasses. The work- 

 manship of the mechanical parts, however, is very good." 



