NEW OIL IMMERSION OBJECTIVES, ETC. 3 LI 



"The numerical aperture of the object-glass (accord- 

 ing to Prof. Abbe's definition, the product of the sine 

 of angular semi aperture with the refractive index of 

 the medium exposed in front) is brought to the num- 

 ber 1.25 exactly, which corresponds to a balsam angle 

 of 113 and is in the ratio of 5:4 greater than would 

 be the maximum aperture, 180 in air, of a dry. lens, 

 considered in its numerical equivalent, the resolving 

 power correspondingly affords a visible increase com- 

 pared with immersion-lenses of the common system, 

 which generally do not exceed 1.10 in the numerical 

 equivalent of aperture. 



" As to the immersion-fluid a large number of ex- 

 periences has shown the oil of cedarwood (ol. ligni 

 cedri) to be the most fitted, though it is in a slight 

 degree still less refractive than ordinary crown-glass ; 

 other liquids of higher refraction exceeding too much 

 in dispersive power. This oil (of which a sample will, 

 be sent with the objective) may be got anywhere in 

 sufficient purity. 



"For controlling its qualities or those of other 

 liquids, which perhaps might appear convenient, a, 

 special test-bottle is forwarded with parallel plane faces, 

 and a prism of crown-glass, cemented to the stop of. 

 this bottle. The vertical spar of a window seen, 

 through the prism and through the oil beneath, should^ 

 appear without a sensible defection and should show A 

 bodies only slightly colored, if the liquid has the right, 

 quality. 



"The pure oil, ligni cedri, will afford the best color- 



