126 DR. W. ROSENHAIN 



A series of papers dealt with the use of the microscope 

 in metallurgy. The subject was introduced by Dr. W. 

 Rosenhain, F.R.S. 



In view of the lateness of the hour, there will not be time for me 

 to read the paper which I have prepared ; therefore I will only deal 

 with one or two points which I think are more relevant to the aspects 

 of the whole question which have already been discussed. I should 

 like to say one or two words with regard to the question of increased 

 magnification and increased resolving power for metallurgical work. 

 There can be no question that we are dependent to a large extent 

 for further progress in certain directions in metallography on obtain- 

 ing higher resolution and higher magnification, but it has been clear 

 to many of us for a long time, and to those to whom it has not 

 been clear it will be so after having listened to these discussions, that 

 magnification alone is quite useless, and that what we must look 

 for is higher resolving power. Mr. Barnard has emphasised the 

 theoretical possibilities of using a much shorter wave-langth. No 

 doubt in the future it may be possible to do that, and Mr. Barnard 

 himself has been singularly successful in utilising the short wave- 

 length of invisible light for photoniicrographic work on transparent 

 sections. About seven or eight years ago I was able to obtain at 

 the National Physical Laboratory a complete outfit of Zeiss apparatus 

 for this purpose, and I spent a large amount of time — over a year — 

 in endeavouring to use it for metallographic purposes, but the result 

 on the whole was extremely disappointing. I succeeded in getting 

 a few photographs, but the time occupied and the labour involved 

 were enormous, and when I did succeed it was only with moderate 

 magnifications. The attempt to use high power monochromatic 

 immersion lenses failed entirely, owing to the fact that I always got 

 milky images. Fluorescence occurred whenever the ultra-violet light 

 struck any object within the tube. When the beam of ultra-violet 

 light has to be sent through a reflector and through the objective, 

 fluorescence occurs on the objective itself, and as a result the light 

 reflected from the back of the objective all over the tube — the actual 

 visible lis^ht due to that fluorescence — became very serious in its 

 actinic effect on the photographic plate, and I felt the only possi- 

 bility of proceeding at all would be if a filter could be obtained 

 which would exclude visible light and transmit the ultra-violet light 

 almost undiminished. Prof. R. W. Wood, of Baltimore, suggested 

 the silvering of one of the lenses, but that increased the exposure 

 so enormously that it was hopeless. Other circumstances arose, and 

 the matter had to be left aside. I hope someone mav succeed in 

 overcoming these difficulties, but I am not sanguine of the results 

 which can be obtained with any kind of invisible radiation, and my 

 reason is that such methods will only yielH photographs. Photo- 

 graphs are extremely useful as a record of w^hat you have seen, 

 but as a means of actual microscopic examination they are not 

 satisfactory. I always think it is necessary to examine successively 

 large areas, and that you cannot, by using a few photographs of 

 small areas, form a really good opinion. 



