800 Notices respecting New Boohs, 



between 1 cm. balls up to an alternative spark-gap of 5 cm., 

 giving a range of from 2000 up to 50,000 volts across the 

 tube. At the higher voltages a fair amount of ionizing 

 radiation was emitted from the window. By absorption in 

 lead and platinum screens this was found to have an absorba- 

 bility corresponding to the X rays to be expected from such 

 a tube, Although very long runs were made with the tube, 

 no evidence was obtained, by any of tlie three methods, of a 

 radiation capable of penetrating *005 cm. of lead. The 

 present experiments, therefore, have not given any evidence 

 of the existence of particles of the nature anticipated. 



In conclusion, I desire to express my sincere thanks to 

 Sir Ernest Rutherford, to whom, as is obvious, this work 

 owes its inspiration. 



Cavendish Laboratory, Cambridge. 

 July 1921. 



LXX. Notices respecting New Books. 

 The Formation of Images by Optical Instruments. (Edited by 



M. Von K-ohr, translated by E. Kanthack.) Published for 



the Department of Scientific and Industrial Research by His 



Majesty's Stationery Office, 1920. 45s. net. 

 ^TVEIS work, which can be obtained from any of the branches of 

 -■- H.M. Stationery Office, continues the services of the Eesearch 

 Department to Optical Science, which they began, on the side of 

 publication, with a translation of Grleiehen's work. The present 

 work has long been the standard one throughout the complete 

 domain of Geometrical Optics in its more complete aspect, and 

 the only one which gave a really satisfactory account, within the 

 limits of present knowledge, of the aberrations of optical systems 

 as they present themselves to the practical designer as well as to 

 the theorist. The translation is done with great care and not too 

 literally, so that the meaning of the original is never ambiguous. 

 Much improvement on the original has been introduced by change 

 of symbols in certain instances, in accord with more familiar 

 English usage. 



As readers will be aware, the original consists of separate 

 chapters of different authorship, each author being au expert ; 

 together with a general introduction regarding the history of 

 the conception of the work, written by Czapski. This valuable 

 historical document is also translated. 



The Editorial Committee has clearly exercised great care in 

 every particular, and the Department is to be congratulated on 

 the completion of a very significant contribution to the advance of 

 Optical Science and of its corresponding industry in this country. 

 This work, now available in our own language, should have the 

 effect of turning the attention of students much more definitely to 

 the possibilities of this subject than previous works have done. 

 Ifc is produced in au excellent form by H.M. Stationery Office. 



