NEW OIL IMMERSION OBJECTIVES, ETC. 313 



graphic work; since rock-slices for inspection with the 

 microscope will become perfectly transparent, using" 

 the oil without needing a polished surface or a cover- 

 ing-glass cemented on, and may be observed to a 

 greater depth than would be accessible to the higher 

 powers of the ordinary system. 



" In every case the new object-glass, not considering 

 the greater optical capacity in bringing out difficult 

 structures, will prove exceedingly convenient for use, 

 from dispensing with any trouble in finding the right 

 correction. 



4 * Of course the full performance of the increased 

 aperture can be effective only on preparations which 

 are mounted in balsam (or any other medium exceed- 

 ing 1.25 in the refractive index) or which, if mounted 

 dry, perfectly adhere to the covering-glass. On objects 

 separated by air from the covering-glass the lens will 

 not work better than any good immersion-objective 

 with an aperture equivalent to an air-angle of 180. 



" Besides this, for displaying the full performance in 

 oblique light, the illuminating-apparatus must yield 

 pencils of greater obliquity than are directly accessible 

 to a slide from air. The most simple devise forgetting 

 light on such an obliquity without needing a special 

 apparatus (immersion condenser) is a plano-convex 

 lens cemented to the under surface of the slide by a 

 minimum quantity of oil. The ordinary mirror of the 

 microscope brought to a moderate distance from the 

 axis will now yield pencils of any wanted obliquity ; a 

 lens fit for this use will be added to the object-glass. 



