Focometry of Diverging Lens- Combinations. 511 



we obtain by induction an excess of electricity upon the far 

 surface B of the rotating disk : this excess in its turn is con- 

 veyed by the brush and its holder to the next inductor C, which 

 in its tarn repeats the operation, and so on with each in- 

 ductor, for the inductors are, as you see, situated alternately, 

 the first upon one side, and the next upon the other side of 

 the rotating disk. These excesses of electricity seated upon 

 the rotating disk, opposite to the inductors, may be viewed 

 as wave-crests, while the corresponding depressions are under 

 the surface of the inductor. All that is done by the machine 

 is to produce this wave-action in the electrical coating or 

 film ; for there is no metallic connexion between inductor and 

 inductor, nor between the machine and the earth : moreover, 

 all the inductors are charged with electricity of one sign, 

 although, probably, the potential in one inductor may be 

 slightly different to that in the other. 



The alternations are possibly caused by the repulsion 

 between the electrical charges upon the disk and the inductor, 

 and the consequent slipping of the electrical film upon the 

 inductor in such manner as to produce a break in the phase 

 of the wave in relation to the brushes. 



LXIV. The Focometry of Diverging Lens- Combinations. 

 By Professor A. Anderson, M.A* 



ON reading the abstract of a paper by Prof. S. P. Thomp- 

 son, " On the Focometry of Lenses and Lens-Combi- 

 nations, and on a new Focometer," in the ' Proceedings of 

 the Royal Society,' vol. xlix., no. 298, it occurred to me that 

 the same principles are applicable to the case of diverging, 

 or positive, combinations, and that the method might be 

 modified so as to make it capable of measuring the focal lengths 

 and finding the positions of the principal points of such 

 combinations. 



S ± Fl 5/ H x H a VA F a S z 



Let light be incident from right to left on the surface A at 

 a small distance from its vertex, the direction of the light 

 being inclined at a small angle to the axis, and let F x and F 2 

 be the principal foci. Then, distances being measured to the 

 right from Hi and H 2 , the principal points, we have the 

 ordinary formula 



v u ~f 

 * Communicated by the Author. 



