226 DISCUSSION AT SHEFFIELD 



of other stands, including the Zeiss-Martens, but had not seen one 

 to equal the Watson. Opinions differed as to the relative wearing 

 qualities of British and German stands; there was no doubt, however, 

 that the better quality British stands were good instruments, and 

 would stand a great amount of use; at the same time they could 

 be improved by using more suitable kinda of metal for the moving 

 parts, such as pinions and racks. 



With regard to objectives, the English achromatic lenses worked 

 perfectly, providing they were used with yellow-green light, and 

 with low and medium powers one could obtain results comparable 

 with those given by Zeiss apochromats. For low power work they 

 had the advantage' of possessing a much flatter field than the apo- 

 chromats, but they did not work well with blue-violet light. Some 

 of the new apoohromats made by Watson and Swift were, he believed, 

 very good lenses, but he had not tried them. For the highest powers 

 the apochromat was much superior to the achromat, though good 

 results could be obtained with the latter. 



Several remarks were made about fine focussing arrangements. 

 Nearly all his work had been done with a vertical camera, and, being 

 endowed with a rather long arm, he had not needed any extended 

 arrangement for focussing. The arrangements he had seen have been 

 rather a nuisance, and probably the worst was the one fitted to the 

 Zeiss-Martens stand. 



For veiy low power work he did not think any ordinary type of 

 microscopic objective suitable if one required a large field. Some 

 type similar to the Zeiss projectioij lens was much better; with such 

 a lens one could easily obtain a field up to ^ in, diameter. 



With regard to the use of prism or disc illuminators, in spite 

 of what had been said, he believed the disc was very much better 

 than the prism for high power work. Providing the structure was 

 switable and the detail in the section arranged in the right direction 

 (that is, with respect to the prism), one could obtain very good 

 photographs with the prism illuminator, but in most specimens, for 

 example, of pearlite, the laminae were arranged at different angles 

 in various part of the field, and it was impossible to arrange it so 

 that each set of laminae was in the best position to be resolved. For 

 low power work there was no doubt that the prism was superior 

 to many individual discs on the market for the reasons given in 

 his paper. 



Reference was made in one of the papers to the impossibility of 

 obtaining good contrast with medium power dry objectives, such, as 

 the l/6th inch, owing to flare due to reflection of the incident light 

 at the front surface of the objective. He had used a Zeiss 4 mm. 

 apochromat for some years for metallurgical work, and found it 

 quite easy to obtain sufficient contrast. It would be very incon- 

 venient to have to use an immersion lens for such powers. 



Several references had been made during the discussion to the 

 various types of metallurgical stands. Many of the new fancy stands 

 were no improvement on the old type, and very often they were 

 much worse. In a metallurgical stand, the stage should have a 

 coarse adjustment, but the fine adjustment should be on the tube. 

 In any case, the milled head for the fine adjustment should not be 

 fixed to a movable part of the stand (such as the stage), as, if so, 



