886 RECORD OF CURRENT RESEARCHES RELATING TO 



water, he obtained the precise numerical aperture 1*33 (= double 

 the " critical angle," 62° 58', from crown glass to water), &c. With 

 air as the external medium at the plane-front of the lens, the 

 num. ap. 1 • was exactly shown (as, indeed, it is with all immersion 

 objectives having num. ap. greater than 1*0, i. e. greater than cor- 

 responds to the maximum air-angle, 180°). 



In applying this kind of front lens to the water - immersion 

 system, Messrs, Powell and Lealand have distinctly had in view to 

 extend the aperture to the maximum ivith water as the immersion 

 medium. The new ^ has an aperture so near the limit (123° out of a 

 possible 126°),* that it may be taken to exhaust the problem of 

 aperture — so far as it can be exhausted with the condition that the 

 aberrations must be corrected with icater as the inter-medium, and with 

 that initial power of magnification. It is to be hoped that a similar 

 aperture will be obtained with a much higher initial power of magni- 

 fication — say, \, yV, Tijj ^^^ sVj which will practically close the water- 

 immersion question until new refracting media are experimented 

 with. 



There can be no doubt that the development of the homogeneous- 

 immersion system is the problem of the future as regards attaining 

 the limit of visibility with the Microscope. In view of the success 

 that has attended the construction of the new \ water-immersion, 

 with a front lens greater than a hemisphere, Messrs. Powell and 

 Lealand have not hesitated to engage themselves to construct a 

 -^V on a similar formula, but for homogeneous immersion." The 

 objective has since been completed, and has an aperture of 142° 

 (measured in a crown glass semi-cylinder of mean index 1 • 5 nearly), 

 with a focal distance of • 007 inch. 



Penetration. t — Dr. Blackham protests against objectives with 

 penetration, the amount of which he contends increases with the 

 amount of spherical aberration in the objective which has been left 

 uncorrected, and decreases in proportion as the corrections for sphe- 

 rical aberration approach perfection. Penetration, he maintains, pro- 

 diices a melting together or con-fusion of the images and a necessary 

 loss of definition ; and he appears to consider Dr. Carpenter's recom- 

 mendation of focal depth in objectives as inconsistent with his state- 

 ment that the " defining power of an objective mainly depends upon 

 the completeness of its corrections, ... an attribute essential to the 

 satisfactory performance of any objective, whatever be its other quali- 

 ties." He also combats the suggestion that as the human eye has con- 

 siderable penetrating power, that quality must also be good for 

 objectives. He points out (1) that the eye is in fact possessed of 

 penetrating power to a much less degree than is generally suj)posed, this 

 being confounded with the power of accommodation, by means of which 

 the eye can be successively focussed with great rapidity upon objects 

 at different distances ; and (2) that the optical conditions in regard to 

 the relative distances of the object and image being reversed, it does 



* Or \\?P and 122° if the index is taken as 1-52. 

 t 'Am. Journ. Micr.,' v. (1880) p. 145. 



