The President's Address. By Prof. P. Martin Duncan. 1 53 



facture by the construction, by Messrs. Powell and Lealand, of a 

 homogeneous-immersion objective of the large aperture of 1-47 

 N.A. out of a possible 1"52. As long ago as 1850 one of my 

 predecessors in this chair, expressed the behef 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 possi- 

 bilities of the future, it must be admitted that so far as regards 

 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 objectives 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 remembered 

 that it would be essential at the same time to provide an immersion 

 fluid, slides, cover-glasses, and illuminators of the same refractive 

 index as diamond also. 



Penetrating Poiver of Objectives — Dej)th of Vision. 



This again is a subject which has long been obscure; very 

 various opinions being held as to the true nature of what has been 

 generally termed the " penetrating power " of an objective. By 

 some it has been declared to be a defect in the construction of the 

 objective — residual uncorrected spherical aberration, in fact; and 

 by others as necessarily inconsistent with perfect definition, even 

 with the best methods of construction ; the only approximately 

 correct notion regarding it, being that it decreased as the angle of 

 aperture increased. 



Professor Abbe, however, in a very valuable paper, placed the 

 question on the scientific basis so long needed, showing that the 

 total depth of vision in the Microscope, i. e. the solid space which 

 at one focus of the Microscope is visible with sufficient distinctness, 

 depends not merely on the depth of focus of the objective, but is 

 the sum of that and the depth of accommodation by the eye. 



The depth of focus (other conditions remaining the same) 

 varies in inverse ratio to the magnifying power and also to the 

 numerical aperture of the objective. Thus with a j-inch and 

 |-inch of the same aperture the depth of focus of the former would 

 be twice that of the latter, or if the powers are the same but the 

 apertures are '50 N.A. and 1 "50 N.A., it would be as 2 to '66. 



The depth of accommodation depends upon a point which was 

 entirely new to microscopists until developed by Professor Abbe, 

 viz. the peculiar property of microscopical amplification, by virtue 

 of which the linear amplification of the depth of an object is 

 largely exaggerated, being equal to the square of the linear 



