THE PAST AND FUTURE OF THE MICROSCOPE 199 



on which further practical advances were based. At 

 that time, under Abbe's direction, Zeiss was making 

 objectives from previous computations, with only one 

 slight subsequent trial adjustment in the case of the high 

 powers. There were rigid tests of the refractive indices 

 of the glasses, and of the attainment of the exact curvatures 

 of the component simple lenses. This process is now 

 followed by all makers of objectives, and thus the chance 

 of getting a particularly poor objective is eliminated. 

 It is said that the calculations and ray tracings necessitated 

 on merely changing the particular meltings of glass from 

 which the lenses of an objective were made for new meltings 

 of about the same compositions may require a fortnight's 

 work. Hence a maker will keep in stock sufficient of 

 each melting to last for a long time. Hence also, after 

 some years, when one of the lenses of an objective requires 

 to be replaced, it is not possible to do this satisfactorily, 

 since all the glass of that melting is probably exhausted, 

 and a recalculation has been made. 



The Oil-immersion Objective. — In 1878, Abbe embodied 

 a new method of correction of his own in the oil-immersion 

 objective of high aperture. It is true that a few others 

 had tried oil or balsam as an immersion fluid, but none 

 of these objectives were practically employed. In the same 

 year, Koch made use, in his work on bacteria, both of 

 Abbe's new oil-immersion objectives and of his wide-angled 

 immersion condenser. His example influenced all subse- 

 quent bacteriological work. The oil-immersion objective 

 rapidly displaced the water-immersion objective in medical 

 schools and universities; and the uncorrected high-angled 

 condenser, used dry, slowly displaced the concave mirror, 

 on the Continent; and, by imitation, in England and the 

 United States. 



The Apochromatic Objectives. — In 1886, Abbe finally 

 brought out his masterpiece — the apochromatic objectives. 

 In these, he used the new glasses which he had been instru- 

 mental in getting made at a glass works established in Jena 

 for research; research which, as everyone knows, was in 



