DISCUSSION AT SHEFFIELD 229 



found the " Pointolite " extremely useful and, if properly used, it 

 gives excellent results. It gives a very steady and strong illumina- 

 tion, and may be used for everything except screen projection, when 

 the greater power of the arc lamp is required. 



I think rather too much has been said to-night with regard to 

 photomicrography, and too little about the use of the microscope 

 for purposes of examination and study, especially at high powers. 

 Actual research work is not done by examining photomicrographs, 

 but by prolonged visual inspection of structures under the micro- 

 scope. It is rather the tendency nowadays to take a photograph 

 and hardly look at the specimen at all; but, after all, photographs 

 are only imperfect illustrations necessary for reports and publications. 



As regards the present indifferent construction of microscopes, 

 I consider that it is for the metallurgical engineer, who should 

 know what he really requires, to design the mechanical details of 

 his microscope, leaving the optician only to deal with the optical 

 system. Further, with regard to existing microscopes, I quite agree 

 that English microscopes have been very unfairly condemned in 

 comparison with Continental instruments, although as regards objec- 

 tives English makers do not seem to be able to keep up to the same 

 standard of excellence as Continental makers, but they occasionally 

 turn out lenses which are as good as can be obtained anywhere. 

 Then, as regards enlarged photomicrographs, I must say that I fail 

 to see any point in enormously high magnifications obtained by this 

 means. Such photomicrographs at, say, 5,000 diameters magnifica- 

 tion, give no information which cannot be obtained from a photo- 

 micrograph at 1,000 diameters. 



Dr. F. Rogers : As an enlarged photograph it is of some use. 



The Chairman : Yes, as a picture for hanging on a wall. You 

 can, of course, go up to enormous enlargements by the use of a 

 lantern, but this only assists by permitting more people to see the 

 photograph at one time. 



Mr. T. G. Elliot: A photograph taken at 5,000 magnifications 

 has this advantage over an enlargement, that before taking the 

 photograph, you select your field at this magnification, and, as we 

 say on page 5 of our paper, " we consider this an important point." 

 We quite agree with Mr. Dickenson and other speakers who have 

 criticised the use of very high magnifications, that nothing new has 

 been learnt from them ; although we submit that inasmuch as they 

 do enable one to see the details of the structure easier, they have 

 this important advantage over photographs taken at lower magnifi- 

 cation. It was partly because Sir Robert Hadfield believed that 

 we had got as far as was practicable with the apparatus available, 

 that he suggested a Symposium on the Microscope, in the hope that 

 it would focus attention on this point and lead to increased effort 

 to obtain apparatus which would open up new fields of investigation. 



Dr. F. C. Thompson, replying to the discussion, said that as 

 the wave-length of light decreased the resolving power was increased. 

 TJp to the present ultra violet lisfht had been unsatisfactory with 

 metallurgical specimens, though there was no obvious reason why 



