160 PJIOTOMICROGRAPHS OF STEEL AND IRON 



with the clear glass screen and the aid of the focussing magnifier. 

 There is one point that has not been mentioned, which is quite obvious, 

 and that is the necessity that the mounting of the whole photomicro- 

 graphic apparatus should be perfectly rigid and free from vibration. 



We have selected a few photomicrographs in order to show the 

 effect of increasing magnifications on the same section, and also to 

 illustrate well-known types of microstructure at high magnifications. 

 The objectives used in obtaining photographs at 1,500 and over are 

 stated on the plates. 



The photographs on plates A, B, C and D have already been dealt 

 with in the text. 



PLATE E. — Figs. 12, 13, 14 and 15, show the microstruc- 

 ture of a Nickel Chromium Alloy Steel in two different conditions. 

 Even at 1,500 magnifications the structure is seen to be very fine and 

 close textured ; it is rather more clearly defined at 5,000 magnifications, 

 but a structure of this kind is very difficult to photograph owing to 

 the want of contrast obtained even with the most careful etching. 



PLATE F. — Figs. 16, 17, 18 and 19 show the structure 

 of Grey Cast Iron. The black constituent is Graphite, and the ground 

 mass Pearlite. The four photographs on this plate illustrate very 

 strikingly the advantage of higher magnifications in order to see 

 clearly the details of a fine Pearlitic structure. 



An additional photograph (Fig. 19a) is given on Plate F, which has 

 been obtained by making an enlargement of the negative from which 

 Fig. 18 was obtained. The enlargement has been so adjusted 

 that its magnification is 5,000 ; a comparison is therefore 

 obtainable with that of Fig. 19, which has been obtained by 

 the direct method. There does not appear to be much to choose between 

 the two Photographs in this instance, but in the case of more com- 

 plicated subjects such as those illustrated in Figs. 20 to 22 on Plate G, 

 the direct method of photomicrography, although very much more 

 difficult than the indirect one of enlargement, is far preferable to the 

 latter because the choice of field to be photographed is made at high 

 magnification — an important advantage. 



PLATE G. — Figs. 20, 21 and 22. Photographs 20 and 

 21 show the microstructure of a Carbon-Chromium steel in two different 

 conditions, magnified 8,000 diameters. The former is a Sorbitic 

 Pearlite structure, and the latter consists of Martensite and Troostite 

 (black areas). — Fig. 22 shows the microstructure of a quenched 

 Carbon steel at 8,000 magnifications, and is Troosto-Martensite. 



PLATE H. — Fig>s. 23, 24 and 25. These photographs show 

 the microstructure at 5,000 magnifications of a steel containing 

 1.41 per cent. Carbon in three different conditions. Fig. 23 is 

 a typical Pearlite and Cementite structure ; Fig. 24 a Martensitic 

 structure, and Fig. 25 a structure of mixed Troostite and Cementite. 



The value of higher magnification especially as illustrated in 

 Figs. 8, 9 and 19 can be emphasised as a result of this research. 

 These Photographs at higher magnifications show in a striking manner 

 the details of a structure which at lower magnifications are only 



