ME. A. T. ADAM AND MR. F. S. MEKKiLS 197 



The following contributions have been received to the 

 discussion on the paper by Sir Robert Hadfield and 

 Mr. T. G. Elliot. 



Mr. A. T. Adam and Mr. F. S. Merrils: In studying the micro- 

 structure of steel wires we have found it necessary to employ high 

 magnification. The difficulty in resolving the structure of carbon 

 steel wires is due in the first place to the nature of the chief con- 

 stituent in properly heat-treated wire, viz., " Sorbite," or " Sorbitic 

 pearlite," and secondly to the minuteness of the structure caused 

 by cold work. 



Some time ago we were fortunate in securing a very good Leitz 

 1/12 in. oil-immersion achromatic objective, N.A. 1.3, which has 

 enabled us to obtain sharp photographs of wire up to a magnifica- 

 tion of 2,500 diameters. This we have found to be about the highest 

 magnification at which good definition and detail are retained with 

 this objective. In certain special cases we have gone up to about 

 5,000 diameters with distinct advantage. 



One of the contributors, being engaged in an investigation on 

 '' The Relation of Heat Treatment to Cold Work," has foinid these 

 photographs of great service in illustrating the effect of cold work 

 on the structure, and hopes to have them published in the Carnegie 

 Scholarship Memoirs of the Iron and Steel Institute this year. 



It is admitted that there is a certain loss of detail in these 

 photographs as compared with visual examination, but this detail 

 is lost in any photograph where an ordinary eye-piece is used. On 

 the other hand, certain features which are barely visible in a photo- 

 graph at, say, 1,500 diameters, are more pronounced in the enlarge- 

 ment obtained by increased camera length. 



In view of these attempts at high power photomicrography, we 

 are therefore extremely interested in the authors' work in this 

 direction, and we are in entire agreement with them in the belief 

 that there is a great field for further exploration in this direction. 



One or two examples of photomicrographs at high magnification 

 are given below with details. The source of illumination used is 

 a tungsten arc 500 candle-power " Pointolite," made by the Edison 

 Swan Electric Co., Ltd. With this source of light it is only neces- 

 sary to use a single condenser to focus the image of the incandescent 

 arc on to the plain glass illuminator. It may be of interest to add 

 that Wratten and Wainwright colour filters, M series, were used 

 with Wratten M Panchromatic Plates in taking these photographs. 



Fig, 1 shows that it is possible to obtain good definition in a 

 photograph at this magnification. The subject is possibly not one 

 that requires high magnification in itself, but it is useful for pur- 

 poses of comparison with subjects that do require such magnifica- 

 tions, e.g.. Fig. 3. 



Apart from this, it appears to show that ** pearlite " is a more 

 complex constituent than lower power photographs indicate. The 

 contributors have always considered the idea that *'' pearlite " is 

 constructed of alternate layers of ferrite and iron carbide completely 



