Chemistry and Physics. 97 



in accelerating the ions that the first emerging energy is very 

 small in comparison with the incident energy. The preceding 

 statements have been generalized for the case of oblique incidence. 



A curve is given in the second paper showing the form of two 

 "forerunners" and the arrival of the "signal." In another dia- 

 gram three curves are drawn for the region of anomalous disper- 

 sion. Frequencies are laid off as abscissae and the ordinates 

 denote the ratio of the velocity of light in free space to the veloc- 

 ity of propagation in question. Hence, in a certain sense, the 

 ordinates mean indices of refraction. The three velocities involved 

 in the denominator of the ratio just mentioned are the phase, 

 group, and signal velocities. The curves for the group and phase 

 velocities follow the familiar course, having maximum values 

 greater than unity and minima less than 1. The graph corre- 

 sponding to the signal velocity has two maxima and one minimum, 

 but nowhere falls beloio unity. At the minimum the signal 

 velocity attains the value of the vacuum velocity. It is shown 

 that the portions of the group and phase velocity curves which 

 have ordinates inferior to unity do not correspond to physical 

 reality (as velocities) because the underlying definitions break 

 down. Consequently the apparent lack of agreement between 

 the experimental data and the Principle of Relativity has been 

 satisfactorily removed by these two very important mathemati- 

 cal investigations. — Ann. d. JPhysik, vol. xliv, pp. 177-240, May, 

 1914. h. s. u. 



11. Dialogues concerning Two New Sciences; by Galileo 

 Galilei. Translated from the Italian and Latin by Henry Crew T 

 and Alfonso de Salvio. Pp. xxi, 300. New York, 1914 (The 

 Macmillan Co.). — The following sentence, quoted from the trans- 

 lators' preface, shows very clearly why the arduous task was 

 undertaken, namely : " For more than a century English-speaking 

 students have been placed in the anomalous position of hearing 

 Galileo constantly referred to as the founder of modern physical 

 science, without having any chance to read, in their own language, 

 what Galileo himself has to say." The text of the National Edi- 

 tion of the Works of Galileo has been strictly followed. The 

 English translation is enhanced by the inclusion in it of an 

 "Introduction" by Antonio Favaro of Padua, who has devoted 

 the best years of his life to the production of the twenty volumes 

 of the National Edition. Le Mechaniche and De Motu Accele- 

 rato by Galileo have been wisely omitted from the present vol- 

 ume since the former contains little but the Statics which was 

 current before the time of Galileo, and the latter is essentially 

 included in the "Dialogue" of the "Third Day." No pains 

 have been spared by the translators to reproduce as accurately as 

 possible Galileo's thoughts, having due regard, at the same time, 

 to all details which make for elegance of presentation. 



As regards the original text, lack of space precludes the possi- 

 bility of doing justice to the subject. Moreover, the book ought 

 to be read by all who are engaged in teaching physics. There- 



Am. Jour. Sci.— Fourth Series, Vol. XXXVIII, No. 223.— July, 1914. 



7 



