112 PROCEEDINGS OF THE SOCIETY. 



arrangements of condensers which fulfil these conditions. In connexion 

 with the vertical illuminator, while the disc pattern is held to be 

 superior to the prism, the faults of individual discs (owing to the un- 

 suitable nature of the reflector) are pointed out. The influence of the 

 curvature of the back combination of the objective on the production of 

 flare is dealt with, and the differences found in achromats and apo- 

 chromats mentioned. After a general discussion on the use of colour 

 screens, the relationship of aperture and magnification and possible 

 future developments in objectives for metallurgical work, the paper 

 gives a description of apparatus specially designed for obtaining low- 

 power photomicrographs embracing a large field of view. Some typical 

 results obtained are included. 



Mr. Leslie Aitchison, D.Met., B.Sc, A.I.C., and Mr. F. 

 Atkinson read a paper on " Metallurgical Microscopes and their 

 Development." 



This paper is written from the point of view of the working metal- 

 lurgist to whom the microscope is of constant value and usefulness ; no 

 attempt is made to discuss the subject from the optician's point of view. 



Sir Egbert Hadfield, Bart., D.Sc, D.Met., F.R.S., and Mr. 

 T. G. Elliott, F.I.C, F.R.M.S., presented a paper entitled " Photo- 

 micrographs of Steel and Iron Sections at High Magnifications." 



Further progress in metallography depends essentially on the use of 

 more powerful microscopes giving higher magnifications allied with 

 increased resolving power. To this end the authors have carried out an 

 extensive research on steel and iron sections up to a magnification of 

 8,000 diameters. The paper is accompanied by eight plates containing 

 twenty-six photomicrographs, the first two being selected from one of 

 Sorby's earlier papers at nine magnifications, the remainder being 

 obtained by the authors under various conditions at magnifications from 

 100 to 8,000. The type of apparatus required is dealt with and also 

 the precautions necessary for exacting work of this nature, and it is 

 shown in what directions further progress is to be anticipated. 



Mr. F. C. Thompson, D.Met., B.Sc, contributed a paper on " The 

 High-Power Photomicrography of Metals." 



The paper aims chiefly at emphasizing the predominating importance 

 of adequate resolving power for high magnifications. Starting with the 

 fact that it is impossible to produce a microscopical rendering of a point 



other than as a disc, the diameter of which = , where 7n is the 



magnification, A. the -^ave-length of light used, and N.A. the numerical 

 aperture of the objective, it is shown how sorbite may become apparently 

 laminated and how pearlite or sorbite may lose their structure, becoming 

 apparently troostitic. The probable value of the use of ultra-violet 

 " light " and silica lenses is pointed out. The " Davon " super-micro- 

 scope is considered at some length, the conclusion arrived at being that 

 whatever may be its merits in other directions it possesses little or no 

 value for high-power photo-micrography as a result of the altogether 

 inadequate resolving power. 



