The President's Address. By the Bev. W. H. Dallinger. 187 



Speaking from observation I may say that all the objectives I have 

 employed for the most critical work, fail to produce images by the 

 extreme marginal zone of the aperture. It is the judgment of com- 

 petent judges that it will be fair to roughly estimate this defective 

 outermost zone at ten per cent., so that from the total measurement of 

 the aperture by Prof. Abbe's method, I find that in practice this amount 

 may be deducted, as of very little service, in all apertures beyond about 

 1 • 3 ; hence, to be able to utilize fully any given aperture beyond 1 • 3, 

 it is practically necessary that the measurement by means of Abbe's 

 apertometer, should be about ten per cent, higher. 



But a further advantage of great numerical apertm-e is that, other 

 things being equal, we can utilize with excellent results deeper eye- 

 pieces. 



I have long realized the advantage, with finely corrected objectives, 

 of a far larger series of eye-pieces than the catalogues provide. Messrs. 

 Powell and Lealand several years ago made me one or more eye-pieces 

 between each of their deeper eye-pieces of standard catalogued focus; 

 and they certainly, within the limit of excellence, beyond which greater 

 eye-piece power cannot be employed, bring out to far greater perfection 

 the qualities of any high class object-glass. 



But we have had announced to us an improvement of the optical 

 arrangement of the Microscope, based upon an important and funda- 

 mental change in the media employed in the construction of object-glasses 

 and eye-pieces ; it will be known that I refer to the system of apochro- 

 matic object-glasses and compensating eye-pieces devised by Prof. Abbe, 

 and under his auspices, carried out by Messrs. Zeiss of Jena. 



The aim of the construction of these new objectives and eye-pieces 

 has been to provide a higher degree of achromatism than could be 

 reached by the old media ; the new kinds of glass produced at the Jena 

 optical glass works, under the superintendance of Dr. Schott and Prof. 

 Abbe, can be so combined in the construction of an object-glass, as to 

 achromatize not only the essential portion of the primary spectrum, but 

 also to a great extent the secondary spectrum, leaving only small 

 residuals of the tertiary order still visible under certain test conditions. 

 The final elements of correction are supplied by " compensating " eye- 

 pieces of special construction, designed to correct what Dr. Abbe refers 

 to as " the difierences in the amplification of the image for the various 

 colours .... formed by the objective outside the axis, which cannot 

 be corrected in the objective itself." 



The first trials of these new optical combinations made in Germany 

 evoked unstinted praise ; and those who, like myself, desired nothing so 

 much as real improvement, awaited their arrival in England with eager 

 and even anxious curiosity. 



The first that came to this country came to Mr. Frank Crisp, and by 

 his courtesy this lens, an apochromatic of 1/8 in. focus, was placed in my 

 hands. I subjected it to comparison, in succession, with my complete set 

 of high powers, including those of N.A. 1 • 5, and upon tests, and by 

 methods which I have indicated. 



It will be well understood that the high excellence and great 

 aperture of my three latest object-glasses, to say nothing of the very 



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