216 Royal Society : — Lieut. Sale on the Action of Light 



apt to be passed over. The book is interesting in another way ; it 

 is quaint without affectation, and produces a very distinct impres- 

 sion of beiug written by a man of a genial and original character. 

 Here and there little pieces of classical lore are felicitously intro- 

 duced, and the more appropriately as Eton was the scene of Mr. 

 Hawtrey's labours. The dedication runs thus : — " Mvarais suis 

 aycnrr)To~is hoc opusculum dicat S. H." The "initiated" are 

 those pupils who have mastered the fourth proposition — who can 

 not only write it out, but understand and are able to apply it 

 — who having got hold of the key, can go on to investigate the 

 hidden treasures of Euclid. We cordially recommend the book to 

 all who are engaged in teaching the elements of geometry to the 

 young ; they cannot fail to be interested, and will probably learn 

 from it something as to the best way of performing a difficult and 

 important task. 



XXIX. Proceedings of Learned Societies. 



ROYAL SOCIETY. 



[Continued from p. 74.] 



May 8, 1873. — Francis Sibson, M.D., Vice-President, in the Chair. 



r pHE following communication was read : — 

 ■*- " The Action of Light on the Electrical Resistance of Selenium." 

 By Lieut. Sale, R.E. 



It having been recently brought to notice that selenium in the 

 crystalline condition exhibits the remarkable property of having a 

 conductivity varying with the degree of light to which it is ex- 

 posed, the following experiments were undertaken with a view to 

 the further elucidation of the matter : — 



Experiment 1. — A bar of crystalline selenium measuring approxi- 

 mately 1-5" x *5" x '05" was procured, and platinum wire terminals 

 were fastened to the ends. 



The bar itself was then enclosed in a box having a draw-lid, so 

 as to admit or exclude the light at pleasure. 



Then, the lid of the box being on, the resistance of the selenium 

 was measured by means of a high-resistance galvanometer and a 

 Wheatstone's bridge, with dial-coils capable of measuring up to 

 10,000,000 ohms. The battery-power was 2 cells Daniell. 



The measurement was made on a dull cloudy day, and in a 

 room of equable temperature. 



The resistance having been carefully balanced, the lid of the box 

 was withdrawn, when the resistance of the selenium fell instanta- 

 neously and considerably, as indicated by the rapid movement of 

 the spot of light on the galvanometer-scale. 



Experiment 2. — The transition from darkness to the light given 

 by an ordinary gas-burner (conditions as before), caused a slight 

 and barely perceptible fall in the resistance. 



Experiment 3. — The bar of selenium was next tried in the solar 



