NEW OIL IMMERSION OBJECTIVES, ETC. 311 



"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 cedar wood (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 

 bodies only slightly colored, if the liquid has the right 

 quality. 



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



