296 Transactions of the Society. 



inferior to Messrs. Powell and Lealand's dry 1/25. Now, the true 

 principles underlying water-immersion were not understood at that 

 time ; it was thought that the only advantage to be gained in adopting 

 it would be a reduction in the quantity of light reflected at the plane 

 surface of the front lens, and therefore the new construction was held 

 to be not worth further consideration. In America, however, the 

 water-immersion was taken up by Wales and Tolles ; but Mr. Stodder, 

 writing upon the resolution of Nobert's bands in April 1868, thinks 

 it necessary to explain the term water-immersion, which shows that 

 there it was quite a new thing at that time. 



Later in the same year Col. Woodward claims to have been the 

 first to obtain a true resolution of the 15 band (90,078 lines to the 

 inch), and says that the Powell and Lealand 1/25 gave the best results, 

 though among the lenses he was using there were a No. 11 Hartnack 

 immersion (1866) and a Wales immersion 1/10. It would seem that 

 these immersion lenses were of • 98 N.A., and Powell and Lealand's 

 1/25 was close on N.A. 1-0. The immersion objectives by Messrs. 

 Hartnack in the Paris Exhibition of 1867 were much improved. 

 Mr. J. Mayall, jun., writing in October 1868, prefers immersion 

 objectives to dry. 



Early in 1869 Messrs. Powell and Lealand brought out their first 

 immersion, a 1/16, and with this Col. Woodward,* in April 1869, 

 succeeded in resolving the 19th band (112,597 lines to the inch), 

 a performance which all the other lenses in his possession failed to 

 accomplish. 



In July 1872 we first hear of a water-immersion 1/10 having a 

 duplex front made by Tolles. (Mr. Wenham's proposed duplex front 

 for a dry lens in March 1869 may have suggested the idea to 

 Mr. Tolles.) In October 1872, Col. Woodward stated that, with Tolles' 

 new water-immersion 1 /16 (which presumably had a duplex front), he 

 had been able to surpass all his former work, including that done with 

 Powell's 1/16 immersion of 1869. 



In December 1874, Messrs. Powell and Lealand brought out their 

 new formula water-immersion lenses: these had duplex fronts, and 

 were remarkably 'fine lenses. The next step was the introduction of 

 homogeneous immersion by Mr. Stephenson and Prof. Abbe in April 

 1878 ; but it is necessary to point out that homogeneous-immersion 

 lenses had been previously made by Tolles, who used soft balsam 

 instead of oil of cedar as the immersion fluid. In April 1879 Messrs. 

 Powell and Lealand exhibited their first oil-immersion, an 1/8 ; and 

 having now brought the history both of the Microscope objective and 

 of Mr. Powell's connection with it to comparatively recent times, we 

 must stop. 



Powell was famous for his achromatic condensers ; and for many 



* Col. Woodward's claims wore disputed at the time, but anyone carefully reading 

 his masterly articles and the letters of his disputants can only come to the conclusion 

 that the colonel's claims were just. 



