SIR ROBERT HADFIELD, BART. 17 



crystal of iron is a double pyramid upon a flat square base, and that 

 the ratio of height to base of the pyramid is proportional to the carbon 

 content. In Cast Irons and hard Steel the crystals approach the 

 cubical form, whilst in Wrought Iron the pyramids are almost flattened 

 down into leaves. In addition the quality of a steel is shown by the 

 arrangement of the crystals. The highest quality of steel has its 

 crystals in parallel positions with their axes in the direction of the 

 pressure exerted on them in working. An examination of the fracture 

 of a good steel in reflected light shows a series of parallel streaks on 

 the surface, whereas in a bad steel several systems of parallel lines 

 can be seen. 



The presence of segregated material and size of the grain can 

 also be seen under the Microscope. The absence of the former and 

 the fineness of the latter indicate good material. 



The qualities of parallelism of the material and fine grain seem to 

 be due to difterent causes. The former seems to be caused by the 

 action of heat, and repeated melting is the great panacea in this 

 respect, whilst the latter is brought about largely by w^orking the 

 material ; on the other hand working the material seems to prove 

 that parallelism and high temperature bring about the coarsening of 

 grain. 



Singular to say, Kohn does not make a single reference to the work 

 of Sorby, which was evidently then only knowTi by few people. 



Dr. Dallinger, F.R.S., who resided many years in Shefiield, gave 

 in the Journal of the Royal Microscopical Society, Vol. 17, 

 1877, page 224, a " Note on the Ultimate Limit of Vision " as 

 applied to modern Microscopical Lenses. He reasoned that mathe- 

 maticians of the first order, notably Helmholtz, had concluded that 

 the limit of vision had been reached and that the Optician could 

 practically give no further aid. Dr. Dallinger considered that the 

 limit marked out was about the one-hundred-and-eighty-thousandth 

 of an inch, and added that he did not consider the limit of visibility 

 had been reached. 



Dr. Sorby in a Paper on the " Limit of the Powers of the 

 Microscope " to the same Society in 1875 referred to an experi- 

 ment of Dr. Royston-Pigott w^hich showed that the smallest visual 

 angle he could ever distinctly appreciate was a hole 1 J in. in diameter 

 at a distance of 1,000 yards, which corresponds to about 6 seconds of 

 arc. This visual arc in a Microscope magnifying 1,000 linear would 

 correspond to about three-millionths of an inch. 



Tchernoff took up the study of crystallization of Steel, 

 his work being brought before this Country by the late Sir 

 William Anderson. In 1878 Wedding studied Steel by the aid 

 of the Microscope. The work of these investigators caused rapid 

 increase of interest in this subject. 



Dr. Martens of Berlin rendered further valuable services, in fact was 

 one of the first to introduce the practical study of Iron and Steel by 

 Metallography. Martens' w^ork commenced about 1878, when he 

 published notes on the Microstructure of Steel. 



