DISCUSSION AT SHEFFIELD 225 



The Chairman : I am sometimes ashamed of some which I see 

 hanging in my laboratory which are marked " x 117.5," but these 

 were taken 18 years ago, and we have noM^ for many years worked 

 to a few fixed round-figure magnifications. 



Dr. T. Baker: Those of us who have had the opportunity of 

 examining the work of old masters in the art of photomicrography 

 I think will agree that they, with imperfect apparatus, turned out 

 much morp satisfactory work than many of us to-day do with a 

 much more perfect equipment. A great deal depends on the 

 operator, and a closer study of the construction of the microscope 

 would assist him in avoiding the pitfalls into which a good many 

 metallurgical microscopists are apt to fall. There is a great tendency 

 to make the metallurgical microscope too complex; amongst the 

 fittings to be avoided are levelling stages and centering nose-pieces ; 

 a centering stage is much better than the latter, since it can be 

 much more substantially constructed. 



As regards objectives, apochromats are without doubt a valuable 

 asset to the skilled worker, but how many can distinguish between 

 the image formed by a good achromatic and that given by an apo- 

 chromatic objective, without the assistance of the inscription on 

 the mount; then, again, by far the greater part of the work of a 

 laboratory does not call for the use of apochromats. 



As regards magnification, it is generally stated in the standard 

 works on the subject that little if anything is gained by using 

 magnifications greater than 1,000 times the numerical aperture of the 

 objective, so that until the resofving powers of objectives are in- 

 creased there seems to be little advantage in pushing magnifications 

 much beyond 1,500 diameters. 



As an illuminant the speaker prefers the direct current arc to 

 the '' Pointolite " lamp, in spite of the fact that the latter has 

 several points in its favour, such as steadiness and constancy of 

 brilliancy. The prism form of vertical illuminator appears to have 

 fallen into bad repute; the speaker, however, prefers it to the 

 cover-glass type, in spite of the fact that it reduces the numerical 

 aperture of the object by one-half in one direction, a weakness which 

 is not such a serious matter as many try to make out, 



Mr. H. Wrighton said he had considerably reduced the flare in a 

 4 mm. .95 N.A. objective by blacking the inside of the mounts 

 near the front of the objective, which were brightly polished. He 

 produced further photographs of a very fine pearlitic structure, and 

 said he considered that, taken at 8,000 magnifications, was better 

 than the corresponding photograph of the same field at x 1,500, as 

 the details of the structure could be more plainly seen. A Zeiss 

 X 12 compensating eye-piece was used in obtaining the photoprraphs 

 at 8,000 magnifications. He submitted photographs of a long distance 

 fine focussing adjustment he had fitted to his Zeiss-Martens hori- 

 zontal microscope, and found to be quite satisfactory, 



Mr. J. H. G. Monypenny, referring to the capabilities of differ- 

 ent stands, said he had never met one to equal the large " Works " 

 model made by Watson. He had used one of these stands fifteen 

 years, and it was still in perfect condii^ion. He had tried a number 



