LESLIE AITCIIISON AND F. ATKINSON 153 



Connected with the photomicrograph ic outfit is the trouble which 

 is experienced because of the lack of adequate devices for the preven- 

 tion of vibration of the apparatus. The greatest proportion of metal- 

 lurgical photomicrography has to be done in works. Such places are 

 always subject to a fair amount of vibration, which is usually trans- 

 mitted in a greater or less degree to the apparatus. This almost 

 renders high power photography of the higher order impossible. The 

 problem of vibration is not new, and various efforts have been made 

 to solve it by the makers, but up to the present these efforts have not 

 proved at all successful. The existence of the vibration is one of the 

 factors which limits the development of higher powers in industrial 

 microphotography. 



Photomicrography is not possible at all unless the source of light 

 is good and reliable. The arc lamp is not good for photography. It 

 is too uncertain. It is very prone to flicker, and also to give a 

 wandering source. Further than that, it requires a good deal of atten- 

 tion — replacing and adjusting of carbons — whilst it also usually in- 

 volves the use of a mechanical device for keeping the arc in its correct 

 situation. Lime-light does not suffer from these defects, but it 

 presents the other difficulty, namely, that it is not sufficiently intense 

 to provide a good illumination at high powers, and therefore to allow 

 of short exposures. The specially designed sources of illumination, 

 such as the Pointolite, are distinctly better, and it is considered that 

 this is the form of illumination which will best repay development. 



It is obvious, of course, that the improvements indicated above 

 are of little value unless they are accompanied by excellence in the 

 more purely optical parts of the outfit. The critical part of the micro- 

 scope is undoubtedly the combination of the lenses, and it is probably 

 in this part of the equipment that the standard Continental makes are 

 most missed. There are numerous points about the lenses which could 

 be considered, but one of them can be selected as typical. This is 

 the production of a flat field. In this respect the Continental outfits 

 showed a great superiority, as they permitted photographs to be ob- 

 tained perfectly sharp at really low magnification (10). This was of 

 immense importance in the study of cast metals and in watching the 

 persistence of the primary crystallisation of a metal through all the 

 subsequent stages of working and treatment. Unfortunately no such 

 good results can be obtained with the usual British lenses. 



The usual range of magnification of the modern microscope ends at 

 about 1,500 diameters — particularly in so far as photography is con- 

 cerned. Unfortunately there are many things which this magnifica- 

 tion does not reveal, and which the metallurgist would be glad to 

 investigate. It seems that it might be useful to indicate a few of 

 the points which the invention of a " metallurgical ultra-microscope 

 might be expected to make plain. 



Several such points arise in connection with tempered steel, and 

 although it is not possible to make their significance entirely plain 

 without introducing a good deal of matter apart from microscopical, 

 they may be taken as typical problems. As is w^ell known, hardened 

 steel consists essentially of a solid solution of iron carbide in iron. If 

 this solution be tempered, a certain change takes place in it, which is 

 reflected in the mechanical properties of the steel, which becomes softer 

 and tougher. This change is usually (and probably quite correctly) 



