DISCUSSION AT SHEFFIELD 227 



tlie flexure due to the pressure of the hand might be sufficient to 

 affect the focus of a high power objective. In this respect it might 

 be mentioned that when using a 2 mm. immersion objective of 

 N.A. 1.40 a nioveinent of ~J- of an inch along the optical axis 

 was sufficient toi put the field out of focus. 



With, regard to Dr. Thompson's remarks on the halo produced 

 round the fine detail in photographs at very high magnifications, 

 while agreeing that such halos were produced, he thought their 

 width was rather less than stated by Dr. Thompson. 



Mr. Birch pointed out that by the use of different screens 

 photographs were obtained which seemed to represent two totally 

 different things. The whole process of photography should be under- 

 stood besides the optical system. 



Dr. W. H. Hatfield said that with regard to the standardisation 

 of magnifications in his laboratory, they had found it helpful to 

 standardise to 10, 50, 100, 500, and 1,000 diameters. It would 

 be very helpful in studying the w^ork of other people if the photo- 

 graphs were of the same magnifications. 



Mr. J. N. Greenwood: The question of the size of reproduction 

 also arises. 



Dr. W. H. Hatfield: I suggest that the matter is worthy of 

 consideration. 



The Chairman : Such standards have been laid down in America. 



Mr. T. G. Elliot: Sir Kobert Hadfield has taken a great interest 

 in the question of standard magnifications for photomicrographs, and 

 he long ago decided to use standard magnifications in his own re- 

 search laboratory. He took a practical interest in the work of the 

 Committee of the "American Society for Testing Materials," which 

 was responsible for drawing up the report on " Magnification Scales 

 for Micrographs," which has already been mentioned this evening , 

 and several of his suggestions were adopted and are included in the 

 revised report, published in June, 1918. In that year, too, Sir 

 Robert endeavoured to get the British Engineering Standards Com- 

 mittee to take up the subject in this country. After due con- 

 sideration, however, they decided against it, because they felt it 

 w^ould be impossible, at that time, to standardise the lenses to be used 

 in obtaining the magnifications, without which the standardisation 

 of magnifications would be useless. It was also thought that this 

 matter and the related one of the full-sized reproduction of photo- 

 micrographs might well be left to the Publications Committees of 

 the various Societies interested. 



The Chairman said that in his laboratoiy they had adopted 100, 

 250, and 750 as standard magnifications. 



Dr. F. Rogers advocated the adoption of round-number standard 

 magnifications for reproduction and report purposes. 



