560 Notes. 



President dealt with the work of Abbe at Jena, as perhaps the 

 most striking example of the results accruing from the reasoned 

 combination of theory and practice. Sketching Abbe's career, he 

 passed on to his work on the theory of the Microscope, pointing 

 out that it was the direct outcome of the work of Fresnel. Abbe's 

 work soon led him to realise that for Microscope objectives no 

 great improvement could be expected with the glass at the opti- 

 cian's disposal — a result which had likewise been arrived at by 

 Petzval and von Seidel in regard to photographic lenses. Theo- 

 retical work thus indicated a bar to progress only to be overcome by 

 the manufacture of new glasses. This fact had also been recognised 

 by our countrymen, Mr. Vernon Harcourt and Professor Stokes, 

 who had for some eight years previous to 1870 endeavoured, but 

 with scant success, to produce glass having certain definite relations 

 between dispersion and refraction. Abbe was more successful : his 

 writings attracted the attention of the glass-maker Schott, and 

 their researches, aided in the first instance by a large grant from 

 the Prussian Minister of Education, had led to the present well- 

 known industrial results. Nor was this all ; for, in virtue of the 

 distribution of profits settled by the scheme of the Carl Zeiss 

 Stiftung, the University of Jena alone had received a sum approach- 

 ing 100,000/. No better illustration, perhaps, could be found of 

 the way in which progress depended on the co-operation of science 

 and experience. 



A fitting accompaniment to the President's address will be 

 found in the volume of the Proceedings of the Optical Convention, 

 in the shape of an historical chart by Mr. F. J. Chesire, F.E.M.S. 

 This chart contains the names and dates of birth and death of the 

 foremost workers in optical science, and is conveniently arranged 

 to show at a glance the periods of particular progress. 



The programme of the Convention can be classed into two 

 divisions ; the reading and discussion of papers, and a representa- 

 tive exhibition of optical and scientific instruments of British 

 manufacture. The following abstracts of papers which have a 

 bearing on the Microscope are given in the alphabetical order of 

 the authors' names. Those marked with an asterisk have been 

 kindly abstracted for the J.Pi.M.S. by the authors themselves. 



* The Consideration of the Equivalent Planes op Optical 



Instruments. 

 IByl Conrad Beck, F.B.M.S. „ 



The author explained that in all dioptric optical systems there are two 

 ■well-known pairs of planes, known as the principal and nodal planes respect- 

 ively, which, when the media on both sides of the instrument are the same, 

 such as air, are superimposed in one pair of planes possessing the character- 

 istics of both, and are known as the equivalent planes. 



In considering optical instruments, some system of measurement and 



