306 THE PRESIDENTS ADDRESS. 



the upper lens of the condenser and the plane of the object itself, 

 when the former is in exact focus; but their adjustment demands 

 also exceptional skill and patience ; added to which their use 

 greatly increases the risk of damage, either to the object or 

 to the objective, during the process of finding and focussing the 

 former. 



The condensers of this form which I have already seen cannot 

 be described as perfect, but I cordially recognise the praiseworthy 

 attempts which opticians both in this country and abroad have 

 made to improve an instrument which, for certain special pur- 

 poses, seems to me to have a great future before it. I know that 

 it is contended by many, whose opinion I esteem very highly, 

 that it is useless to increase the aperture of substage condensers 

 beyond N.A. TO, because the objectives of our best opticians are 

 incapable of bearing the extra strain. To this I reply that if this 

 be so our present objectives must be set aside for test purposes, 

 and our opticians must be encouraged to produce lenses which 

 shall stand a greatly increased illuminating cone. But is it 

 really true that none of the objectives at present available will 

 stand an aplanatic cone of more than N.A. I'O ? I do not think 

 that this is the case. With respect to ordinary so-called achro- 

 matic oil immersions of from 1*2 to 1*3 N.A., I admit that an 

 aperture of 1*0 will develop all their powers, and in many cases 

 some of their defects as well ; but I possess an apochromatic lens 

 of 3 mm. focus and 1*43 N.A., which, on suitable objects, will 

 not only stand a much larger cone than this, but will reveal 

 additional structure without loss of brilliancy and without the 

 slightest deterioration of image, when illuminated by a cone 

 approaching 1-3 N.A. Nor is my own lens an exception, for at 

 least three of my friends are the owners of objectives by the same 

 maker, and of the same or of shorter focal length, concerning 

 which the same statement may be made with truth. I am 

 afraid it must be owned that the very considerable trouble 

 and patience which are required in order that the undoubted 

 advantages of oil immersion wide-angled condensers may become 

 apparent have deterred many competent observers from perse- 

 vering in their use. 



For myself, I confess that when I look back on the progress 

 which opticians in this country, as well as on the Continent, have 

 achieved within, say, the last quarter of a century — when I 



