Intelligence and Miscellaneous Articles. 



143 



service which will be greatly appreciated by those who are investi- 

 gating the problems of the optical workshop. He has added 

 considerably to the value of the original work, especially by the 

 addition of diagrams elucidating the meaning of the various 

 -symbols and sign conventions adopted. The formulas given are 

 those due to von Seidel, which require little knowledge of 

 mathematics beyond the elementary parts of trigonometry. Even 

 the practical optician with a slender mathematical equipment 

 should be able to acquire without difficulty the power to undertake 

 computations of the kind dealt with in this volume. In any case, 

 Mr. French has provided, under exceptional war conditions, an 

 excellent handbook, which we commend to the serious attention 

 of every student of technical optics. 



X. Intelligence and Miscellaneous Articles. 



ANGLE TRISECTION. BY H. R. KEMPE, M.INST. C.E. 



TN the course of some quite recent geometrical investigations I 

 ■*■ evolved the following arrangement for trisecting an angle 

 mechanically. The arrangement, so far as 1 am aware, is original, 

 is geometrically correct for all angles, and is exceedingly simple. 



Fig. 1. Fig. 2. Fig. 3. 



Fig. 4. 



Fig. 5. 



In fig. 1, A is a transparent disk (of celluloid), and in fig. 2, 

 -B is a straight-edged link or radial lever (also, conveniently, of 

 celluloid). 



