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. 



"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 coveriny-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. 



44 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. 



