THE HIGH-POWER PHOTOMICROGRAPHY OF METALS. 



By F. C. Thompson, D.Met., B.Sc. (Lecturer on Metallurgy in the 

 University of Sheffield). 



I. — General. 



The high-power microscopical examination of metals is a matter of 

 the greatest importance. 



As Prof. Abbe has pointed out, however, and this forms practically 

 the whole of the sermon which the author desires to preach, " empty 

 magnification " unaccompanied by a corresponding resolving power 

 is of no service whatever to the metallographer. 



As is well known, it is impossible to produce a microscopical 

 rendering of a point other than as a disc of definite dimensions the 

 size of which depends on (1) the numerical aperture of the objective, 

 (2) on the wave-length of the light employed, and (3) on the magnifica- 

 tion. The diameter of this spurious disc D = — ^ . where m is the 

 magnification, X the wave-length of the light used, and N.A. the 

 numerical aperture of the objective. It will be at once seen that 

 the image becomes less and less sharp as the wave-length of the light 

 increases and as the ratio of the total magnification to that produced by 

 the objective itself is raised. 



As shown in Fig. 1, a succession of points may thus, if sufficiently 

 near each other, merge into an apparently continuous line, the trao 

 structure of which would never be realised. Sorbite might in this 

 way simulate pearlite. A very beautiful illustration of this effect 

 in a diatom is given by Spitta, " Microscopy," Fig. 3A, Plate I. 

 It will further be evident that a sorbite in which the diameter of 

 the spurious disc exceeds the distance between the carbide globules 

 will appear practically structureless, Fig. 2A, and that pearlite, tie 

 distance between the laminae of which does not exceed tins diameter, 

 will lose its structure. Fig. 2B. The matter, therefore, is of very 

 real practical import. 



Abbe has shown that in the absence of certain diffraction spectra 

 a line may be duplicated, or even rendered as three. Although in 

 metallurgical work such results are unlikely — they would, for 

 instance, double or treble the fineness of a laminated eutectic — the 

 possibility of such spurious effects at very high magnifications should 

 always be borne in mind. 



According to Spitta the limit of magnification to which it is 

 permissible to go with light of wave-length 540 ium- must never 

 exceed 1,000 times the numerical aperture of the objective. 



It would thus appear that the best method of approach to the 

 very high magnifications suggested lies at the present time in the 

 use of ultra-violet " light " of very short wave-length. Since glass 

 is opaque to such vibrations, quartz or fused silica must be used as 

 the optical material of all lenses, vertical illuminators, etc. In 

 connection with metallographic work, since the objective acts as its 

 own condenser, and since no slips or cover glasses are required, such 

 a system is much cheaper than it can be when using transmitted 



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