SIR EGBERT HADFIELD, BART. 21 



If this Fluorite can be used in the manufacture of Glass suitable for 

 High Power Objectives, then the South African source of supply 

 should be borne in mind. It is also stated that Fluorite exists in 

 Canada, and our Honorary Treasurer, Dr. Robert Mond, is inves- 

 tigating this matter, 



The King recently visited the Leicester Works of Messrs. Taylor 

 and Hobson, the f-amous Lens experts. He there saw the instru- 

 ments by which vital errors of a few milHonths of an inch are 

 avoided, and had explained to him the principles of the use 

 in this connection of light interference, which w^as first studied by 

 Sir Isaac Newton in Soap Bubbles. This Firm also make the "Aviar " 

 Lens, which through repeated calculations and readjustments of 

 formulae enabled the British Photographing Aeroplanes to beat the 

 " Archies." 



In a recent number of the " Scientific American Supplement " 

 (August 30th, 1919), a statement is made that in spite of the traditional 

 superiority of the German Optical Industry, during the War their 

 Lenses proved distinctly inferior to those of French and English make. 

 The English developed superior Lenses during and under the stress of 

 the War. 



In a perfect High Power Objective known as Apochromatic, it is 

 desirable this should give :— 



(a) Full Resolution. — The resolution increases with, and is a 

 function of. Numerical Aperture. The number of lines to the inch 

 which an objective w^ill resolve, if perfect, may be calculated from 

 the Numerical Aperture. 



(b) Good Definition. — Which could be magnified by a x 28 

 eye-piece or its equivalent w^ithout breaking down. 



(c) A Perfectly Flat Field. — This is never actually obtained. 

 {(l) Freedom from Chromatic Aberration. 



Achromatic Lenses generally give good definition and their 

 field is often somewhat flatter than in that of the 

 Apochromats. They do not, however, give such good resolution, 

 and are only partially colour corrected. The latter failing makes 

 them much less efficient than the Apochromats for photographic 

 work. 



The foreign 2 mm. Objective used in the Hadfield Laboratory is a 

 very good one of its class. Its Numerical Aperture is 1.3, and there- 

 fore according to the formula of Professor Abbe, should, if perfect, 

 resolve about 92,000 lines to the inch. I have had photographs taken 

 by it which show 85,000 lines to the inch clearly resolved. Its 

 definition begins to break dowTiwith an eye-piece magnification of about 

 15. For an Apochromat its field is very flat, and it is in this respect 

 chiefly that we found it superior to other Apochromats we examined. 

 Its colour correction is apparently perfect. 



It may be added that Messrs. Watsons supplied to the 

 Research Laboratory of my firm a very excellent 2 mm. Objective. 



