SECTIONAL TRANSACTIONS.—A. 431 
comprising many components cemented together, is compared with the 
Cooke lens in its various types. 
For aerial photography during the war a special type of lens was required, 
and the Cooke-Aviar was developed. At this time also the aperture of the 
Cooke was increased to F/3. In recent times still larger apertures have 
been called for, and the Cooke F/2 and F/2:5 and F/1-5 lenses developed. 
Projection lenses for the modern cinema also have had to be of greater 
aperture, and this has led to the development of a series of lenses of apertures 
F/2 to F/1:5. It is noteworthy that all the modern large-aperture lenses 
consist, like the Cooke lens, of separated components, and that the Con- 
tinental lens, with many components cemented together, has practically 
dropped out in the race. 
The anastigmatic Telephoto lens was also first made at Leicester. 
The problems of optical design, and the methods used, are reviewed. 
The trend of photographic optics is towards the production of lenses of 
small focal length and greater speed, largely influenced by the increasing 
employment of small cameras, kinematographs, and finer grained emulsions. 
Leicester methods of manufacture and testing lenses. Interchangeable 
manufacture. 
Dr. L. Stmons and Mr. E. H. Smart.—Demonstration of a model to 
illustrate the classical motion of a diatomic rotator with two degrees 
of freedom (12.40). 
AFTERNOON. 
Visit to British Thomson-Houston Works, Rugby. 
Monday, September 11. 
DIscussIONn on Atomic transmutation :— 
Rt. Hon. Lorp RuTHERFORD oF Netson, O.M., F.R.S.—A review 
of a quarter of a century’s work on atomic transmutation (10.0). 
In 1907 a discussion on the constitution of the atom was held before 
Section A at the meeting of the British Association at Leicester, in which 
the importance of the study of the transformations of radioactive bodies 
was indicated, and the difficulty of explaining the part played by positive 
electricity was emphasised. 
In 1911 clear evidence for the nuclear structure of the atom was put 
forward. It soon became evident that outer electrons played no major 
part in transmutations, and that in order to institute any permanent atomic 
transmutation the structure of the nucleus must be changed. In 1919 
decisive experiments were made. When «-particles were fired in nitrogen, 
a new type of particle appeared—the proton. Photographic evidence 
showed that the capture of an «-particle by the nucleus was accompanied by 
the emission of a proton. The nitrogen nucleus, therefore, of mass 14 and 
charge 7, assimilates an «-particle of mass 4 and charge 2 and expels 
a proton of mass 1 and charge 1. We are therefore left with a nuclear 
structure of mass 17 and charge 8, which is an isotope of oxygen. Other 
transmutations may be similarly checked, remembering that all such changes 
must obey what may be termed general energy conditions. 
Beryllium of mass 9 and charge 4, when bombarded, captures an «-particle 
of mass 4 and charge 2, giving rise to a nucleus of mass 12 and charge 6, 
