158 PHOTOMICROGRAPHS OF STEEL AND IRON 



obtain photographs of Iron and Steel sections at the high magnification 

 of 8,000 diameters. To give an idea of what this means, it may be 

 mentioned that the diameter of the actual field shown in a 3J" circle 

 photograph at this magnification is only .00041" or 1/2460". The 

 actual area of this field examined is .00000013 square inches. The 

 polished section under micro-examination is usually about J in. square. 

 If the whole of this area w^ere magnified 8,000 times it would yield 

 a square about 55 yards by 55 yards, occupying an area of approxi- 

 mately 3,000 square yards, that is to say, not far away from 

 three-quarters of an acre. 



As is well known, the modern Microscope consists of two systems 

 of lenses, the objective and the eye-piece. The objective gives an 

 enlarged image of the object, and the eye -piece further magnifies this 

 image. The high power photomicrographs given in this Paper are simply 

 high magnifications by means of the eye-piece and extra camera ex- 

 tension of the image given by a 2 mm. objective, or in the case of the 

 8,000 magnifications by a 1.5 mm. objective. Whatever may be the 

 quality of the image given by the objective — for example, as regards 

 resolution — that quality is reproduced in the magnified image of the 

 eye-piece. Thus, if the objective gives a blurred image, the blur is 

 simply magnified. In other words, it is just as though a lantern 

 slide were projected on the screen ; if the slide is a good one we get 

 a good picture, but if bad the picture will be no better because it is 

 magnified. The essential aim, therefore, is to get a very clearly resolved 

 image. This needs a special quality or virtue in the objective, and 

 this virtue is called its resolving power. 



For photomicrography at high magnifications, it is specially essential 

 that an objective of high resolving power should be used. The effect 

 of magnification without resolution is well illustrated by Figs. 

 3, 4 and 5 on Plate B. These photographs are all at 600 magnifications, 

 but taken by objectives of low, medium, and high power respectively. 

 In No. 3 the dark ground mass is left unresolved. No. 4 shows 

 some resolution of this dark ground mass, but in No. 5 it is practically 

 completely resolved into its two constituents, Ferrite and Cementite 

 in lamellar form. In the course of a search for a really good 2 mm. 

 oil immersion objective, for photomicrographic research, we found that 

 results obtainable with a moderate Achromat, compared with those 

 obtained with a good Apochromat, showed difterences at least as 

 great as is illustrated in Figs. 4 and 5. 



An illustration of the microstructure of an Annealed Alloy steel, 

 containing .84 per cent. Carbon and 1.12 per cent. Chromium, is shown 

 at four different magnifications in Figs. 6, 7, 8, and 9 on Plate C. 

 Although the resolution of the structure is the same in Figs. 7 and 8^ 

 because the same objective was used in each case, the eftect of the 

 increased magnification is to show in a striking manner the alternate 

 white and dark constituents of the lamellar pearlite. This effect is 

 further emphasised in a photograph of the same section at 8,000 magni- 

 fications, shown in Fig. 9. There is no doubt that this magnification 

 taxed the lens somewhat beyond its capacity; however, the photograph 

 is certainly a good one and worth including, if only to show the limit 

 obtainable with the apparatus available at the present time. 



