1882.] 



MICKOSCOPICAL JOUENAL. 



109 



the initials N. A, are no longer so 

 mystic a symbol as they have been. 

 I understand that many of the opti- 

 cians have decided to use the numeri- 

 cal notation in the future issues of 

 their catalogues, which is a step in 

 the right direction, which we shall 

 hope to see generally followed.' 



Whilst on this subject I may point 

 out how important it is that in ob- 

 servations with high-power objectives, 

 their aperture as well as magnifying 

 power should be stated. Whether a 

 large or a small aperture has been 

 used, may make a very material differ- 

 ence in the value to be attached to 

 the results described. 



The " Homogeneous Immersion " 

 Principle. — The utility of homo- 

 geneous-immersion objectives being 

 established beyond doubt by practical 

 experience, it is interesting to note 

 that the origin of the principle is 

 very fully recognized by Professor 

 Abbe to be due to our esteemed Fel- 

 low, Mr. J. W. Stephenson. 



The two essential points in homo- 

 geneous-immersion are, ist, the in- 

 crease in aperture obtained by the 

 use of a fluid of high refractive index, 

 and 2d, the enhanced optical per- 

 formance arising from the total 

 suppression of spherical aberration 

 in front of the objective. Professor 

 Abbe states that although Amici first 

 applied oil-immersion, he failed to 

 recognize the specific advantage of 

 an immersion fluid being as near as 

 possible in refractive and dispersive 

 powers to the crown glass (i. e. "homo- 

 geneous "). He finished his lenses 

 and then sought for oils and mixtures 

 of oils of various refractive pov/ers 

 for obtaining the best correction. 

 " It was Mr. Stephenson who, in his 

 first communications with me, ex- 

 pressed the opinion that doing away 

 with the anterior aberration would 

 improve the defining power, and 

 especially would afford very favora- 

 ble conditions for further increase of 

 aperture." 



The importance of this system will 

 be appreciated when we remember. 



in regard to the first point (the in- 

 crease of aperture), that the theoreti- 

 cal resolving power of an objective is 

 thereby raised from 96,400 lines to 

 an inch, which is the maximum of a 

 dry objective, to 146,528 the maxi- 

 mum of an oil -immersion objective, 

 the illuminating power being also in- 

 creased from I to 2.25 : while as 

 regards the second point, we are 

 able, by the homogeneous-immersion 

 method, to reduce the problem of 

 correcting a very wide-angled objec- 

 tive to the much less difficult one of 

 correcting an objective of moderate 

 air-angle. Our lamented President, 

 the Rev. J. B. Reade, declared in 

 1870 that "the ghost of aberration 

 will never be entirely exorcised even 

 by cold water." But there appears 

 to be good ground for believing that 

 oil has practically accomplished that 



object. 

 ******* 



Lastly must be noted an important 

 advance in practical manufacture by 

 the construction, by Messrs. Powell 

 and Lealand, of a homogeneous-im- 

 mersion objective of the large aperture 

 of 1.47 N. A. out of a possible 1.52. 

 As long ago as 1850, one of my pre- 

 decessors in this chair, expressed the 

 belief that objectives had then 

 " nearly, if not quite, attained the 

 limit of perfection," and whilst it 

 will be prudent even at this much 

 later date to avoid any assertion of 

 finality in the present, or scepticism 

 as to the possibilities of the future, it 

 must be admitted that, so far as re- 

 gards aperture and resolving power, 

 we have arrived at a point beyond 

 which it will, to all appearances, be 

 difficult to advance, at any rate not 

 without serious restrictions in the use 

 of the objectives. Whilst it might be 

 possible to work front lenses for ob- 

 jectives out of diamond, and so to 

 increase the aperture to 2.5 N. A., 

 and the resolving power to 241,000 

 lines to the inch, it must be remem- 

 bered that it would be essential at 

 the same time to provide an immer- 

 sion fluid, slides, cover-glasses, and 



