PROCEEDINGS OF THE SOCIETY. Ill 



marized. Rigidity is essential. It is shown how the instrument can 

 be designed on the basis of a machine tool, Large working distance 

 between stage and objectives and freedom of movement for the specimen 

 are important. 



The optical requirements are those common to all the most exacting 

 microscopic work, but ever-increasing demands are being made on 

 resolving power, since metallurgical progress tends to the production 

 of material of extremely minute niicrostructure. 



The accessories are of some importance, particularly the illuminator. 

 For visual purposes a source of light behind ground glass or opal is 

 recommended, no lenses or condensers being necessary. A suitable 

 arrangement is described. 



The author finally describes his optical levelling apppliance for 

 mounting specimens with their surfaces at right angles to the optic axis. 



Dr. Rosenhain, in the course of the discussion, pointed out that 

 one method of getting higher resolving power was to use a front glass 

 of higher refractive index than was at present obtainable. It was 

 doubtful whether a satisfactory and lasting glass could be found, but 

 he thought the solution lay in finding or perhaps growing artificially a 

 crystalline substance. 



Professor Cecil H. Desch, D.Sc, contributed some "Notes on 

 the Construction and Design of Metallurgical Microscopes." 



The author begins with some criticistns of the existing types of 

 instrument, which, while well designed mechanically, do not wear satis- 

 factorily. He proceeds to discuss in some detail the principal parts of 

 the instrument. 



The Stand. — A heavy horseshoe foot is recommended on the whole ; 

 other forms are useful for special purposes. The Le Chatelier inverted 

 stand is pronounced flimsy and needing better design, because specimens 

 are quickly examined in it. 



For larger instruments used for photography the ordinary design 

 might be completely departed from and a type of optical bench devised. 



Adjustments. — All racks and screws should be cut in hard, incor- 

 rodible metals or alloys, adopting engineering methods. 



The Body Tiihe should be short and of wide diameter. 



The Stage. — A rack-work focusing movement should be provided. 

 Levelling stages are a nuisance ; specimens should be levelled in other 

 ways. Mechanical movement is essential and rotation desirable. 



The Vertical Illuminator. — The Beck or transparent illuminator is 

 the only suitable form for high powers. 



The Objectives. — Apochromatics are deficient in flatness of field, and 

 the author questions whether good achromats are not to be preferred 

 for photographic purposes, especially with the almost monochromatic 

 colour screens now in use. 



Mr. J. H. G. MoNYPENNY contributed " Some Notes on the Metal- 

 lurgical Photomicroscope." 



The first part of the paper emphasizes the importance of the illumi- 

 nation in the production of a photomicrograph, outlines the conditions 

 necessary for obtaining good illumination, and gives descriptions of 



