CHAPTEE XXIY. 



IMMERSION, APOCHROMATICS, AND APERTURE. 



OCULARS. 



THE two greatest improvements made in our optical 

 appliances within a period of many years have been, first, the 

 introduction of the "homogeneous immersion" system for 

 objectives; and second, the use of a new glass, having different 

 dispersive powers from glasses previously made. The result 

 of the discovery of this new glass is that in the new 

 apochromatic objectives we have a more complete correction 

 for color than ever we had before ; that is to say, the new 

 lenses are corrected not only for two spectrum regions as 

 formerly, but also for a third region. 



The system of immersion is now so much a matter of ancient 

 history that we need only point out the advantages now 

 universally admitted to arise from the latter development of 

 immersion, viz : Homogeneous or oil-immersion. Before the 

 days of oil-immersion trouble arose from the fact that between 

 the object and the objective two media of different refractive 

 powers intervened, viz : Crown-glass (the cover-glass) and air 

 or water. When a substance was found having approximately 

 the same refractive index as crown-glass, and when the 

 objective was practically joined to the cover-glass by this 

 substance, viz : an oil, it is easy to see how great a step was 

 gained, provided always that the object was either in 

 contact with the cover-glass, or in a medium nearly equal 

 in refractive power, and in contact with the cover. 

 More oblique rays passed from object to objective, the 

 illumination was better, working distance greater, and in fact 

 there was improvement in almost every respect. By the 

 immersion system lenses can be produced of larger aperture 



