92 MR. A. C. BANFIELD 



complaints " that the objective has not a flat field " are so common. 

 The objective is computed for a mathematically flat object; if the 

 specimen deviates from a true plane, then definition is bound to 

 suffer. I merely refer to this point because it suggested to me an 

 idea which it may profit some capable mathematician to investigate, 

 which, briefly., is this: — 



It is just as easy to prepare a metal specimen, worked to a 

 definite radius, as it is to work it to a plane. Unfortunately, I 

 am not a mathematician, but I suggest that by adopting some small 

 concave radius for a metal specimen, say 10 mm., it may be possible 

 greatly to improve the metallographic objective. The improvement 

 may possibly take the form of a greatly simplified construction, 

 or it may prove a means of increasing the N.A. of a lens. Person- 

 ally, were I capable of it, I should compute it first of all unachro- 

 matised for use with the well-known Mercuiy line 5461, a powerful 

 source of monochromatic light easily isolated. It could, if it 

 showed promise, be further computed (all fluorite construction) for 

 the powerful ultra-violet radiation at 1851. This would bring the 

 N.A. for a 2 mm. lens to somewhere about 3.5. 



(7) I have suggested the above (under 6) as a possible source of an 

 improved objective for metallurgical purposes, but by working a 

 specimen to a radius, it is possible to compensate an apochromatic 

 or other objective which lacks flatness of field, by applying the well- 

 known sphereometer formula. 



Take a. ruled stage micrometer, focus the centre of the field, and 

 note reading on micrometer drum of the fine adjustment, after which 

 take the reading of the alteration necessary to render the lines at 

 the edge of the field sharp. Then if S is the semi-diameter of the 

 circle in the object plane represented by the field of view, we can 

 immediately say that if the object be given a curvature whose radius 



the field of view will be in focus simultaneously at the centre and 

 margin, d is, of course, the difference between the two readings of 

 the fine adjustment drum. 



A further group of papers dealing with various general 

 aspects of microscope design and construction, presented by 

 M. Eugene Schneider, Professor Alexander Silverman, 

 Dr. R. E. Slade and Mr. G. I. Higson, and Mr. R. J. E. 



Hanson were taken as read. 



