NEW OIL IMMERSION OBJECTIVES, ETC. 317 



doubtless contract with either Tolles or Spencer for 

 objectives on the model of the Zeiss, and fully equal to- 

 the latter in performance, and at figures considerably 

 below those quoted in the Danish catalogue. 



Our experience with the Zeiss oil immersions has 

 neither weakened our appreciation of the screw-collar 

 adjustment, nor our high estimation of wide apertured 

 objectives of American manufacture. 



