148 SOME NOTES ON THE METALLURGICAL 



to be of flint glass. With metallurgical work, however, these diffi- 

 culties would not occur, and it seems to the author that such a 

 lens would be of value in elucidating some of the finer structures 

 met with in metals. If such a lens could be made it should prefer- 

 ably be apochromatic, but, if not, it might be advisable to correct it 

 for blue violet, as the " preferred colour," in order that the highest 

 resolving power could be obtained photographically. 



It is obvious that at the highest powers the apochromatic lens 

 has a much greater resolving power than the semi-apochromatic of 

 the same aperture, owing to its capability of working with blue violet 

 light. It should be emphasised that, other things being equal, using 

 light of wave-length 4,500 A.U. instead of 5,500 A.U. is equivalent 

 in its effects to increasing the N.A. approximately 25 per cent. 



(e) Eximsure and Vibration Effects. — In metallurgical photo- 

 microscopes for use in works' laboratories it is very important 

 that the exposure required, especially with high magnifications, 

 should be as short as possible in order to avoid the effects of 

 unavoidable vibrations. For such purposes an intense illuminant 

 is required, and such lamps evolve a very considerable amount 

 of heat, which may easily cause trouble with the cement used in 

 the various combinations of the objective. It is very necessary in 

 such cases that an adequate cooling trough be placed in the beam 

 of light before it reaches the microscope. The heat evolved also 

 causes trouble owing to the expansion effects produced in different 

 parts of the microscope and camera. 



Even when the exposures are comparatively short (e.g., a few 

 seconds), they still give plenty of time in the case of the higher 

 powers for vibration to have considerable effect. The author has, 

 however, been able to overcome this completely by swinging the 

 whole photomicroscope on springs, as shown in Fig. 7. It will be 

 noticed that the author's camera is vertical. This position has 

 several advantages from a works' point of view ; obviously it occupies 

 less floor space than the horizontal pattern, and is probably more 

 easily swung than the latter. It may be mentioned that, with the 

 suspension system used, photographs at 1,000 and 1,500 diameters 

 were successfully taken, although the laboratory was within 50 yards 

 of four 8-ton steam hammers, and also adjoined three sets of 

 railway lines running into the works. 



(f) Low Fower Photography. — It is frequently desirable to be 

 able to reproduce at low magnifications fairly large areas under 

 vertical illumination. With ordinary low power objectives {e.g., 

 2 in. or 3 in.), it is possible to take photographs at, say, 20 or 30 

 diameters, but in general the field is only small, about \ in. or 

 tV in. diameter. If attempts are made to get a larger field, 

 trouble is at once experienced with the illumination, and often 

 with the definition falling off. Frequently a very large field is 

 required if the photograph is to s.erve its purpose, as, for example, 

 with groups of flaws, very coarse structures, and segregated areas. 



Some ten or twelve years ago the author devised an arrangfement 

 for this purpose, and as he has found it exceedingly useful, he 

 puts it forward in the hope that it may be of use to others. There 

 is nothing really novel in the method, it is a combination of several 



