The Presidenfs Address. By Prof. P. Martin Duncan. Ill 



Subsequently Professor Abbe sent a communication to our 

 Joui-nal which pretty well sets at rest the former debates about the 

 value of objectives with a high numerical aperture, and thoroughly 

 defines the relative value of the two classes of lenses. He states 

 " the obvious inference ... is that the widest possible apertures 

 must be used for the observation of objects or structures of very 

 minute dimensions, low and moderate apertures for relatively large 

 objects." The ordinary microscopical investigation of the structures 

 of animals and plants can be best done with lenses of low numerical 

 aperture. But when minute structures require to be accurately 

 defined there must be a wide aperture and also a corresponding 

 amplifying power. Deficiency of power renders a high numerical 

 aperture useless, and therefore wide apertures are necessary when 

 a high amplification is required. Professor Abbe's conclusion 

 regarding the practical value of the two classes of objectives is quite 

 consonant with the opinions which have often been expressed at 

 the meetings of the Society : " Wide apertures (together with 

 high powers) for those preparations only, which do not require per- 

 ceptible depth of vision, i.e. for exceedingly flat or thin objects, and 

 for transparent objects which can be studied by optical sections. 

 Moderate and low apertures when a wide range of penetration 

 cannot be dispensed with." 



With great wisdom Professor Abbe explains the positive damage 

 connected with the use of unnecessarily wide apertures and notices 

 that increase of aperture may be prejudicial to the ease and con- 

 venience of microscopical work. It necessitates a progressive 

 reduction of the working distance of the objective, and this is still 

 further diminished during the necessary correction, for increase of 

 aperture is inseparable from a rapid increase of sensibility of the 

 objectives for slight deviations from the conditions of perfect correc- 

 tion. He impresses upon microscopists that the best wide-angled 

 system, if not carefully adjusted when in use, is not better than a 

 bad low-angled lens ; for the tolerably sharp image which could be 

 still obtained through the central part of the aperture only (even 

 under the imperfect state of correction) is disturbed by the coarse 

 dissipation of light from the ineffective marginal parts of the 

 aperture. Dividing microscopists into those who amuse themselves 

 and those who work, Professor Abbe advises the latter never to use 

 wider apertures than are necessary for the efiectiveness of the power, 

 because excess of aperture always involves waste of time and labour. 



It has always seemed an anomaly that when a good objective 

 with a high numerical aperture has been obtained, many operators 

 have diaphragms or stops added. The object of so doing 

 is evident, but the practical dijQficulty arising from the small 

 working distance is not removed. Moreover, there is a reason- 

 able doubt whether an objective thus treated is really as efi'ective as 



Ser. 2.— Vol. III. N 



