October 2, in03.] 



SCIENCE. 



433 



scientific journals and have hence been inac- 

 cessible to many who might otherwise desire 

 to study them. Hence the gathering together 

 into one volume of the main points of that 

 argument, together with a discussion of how 

 they have been used in the establishment of 

 that theory and of the present tendencies and 

 possibilities of that theory, is performing a 

 great service not only to those who desire to 

 learn of the theory, but also to the theory it- 

 self, since such work must help to show where 

 modification and extension of the theory are 

 possible and desirable. 



It was a desire to meet these needs both of 

 the student and of the theory that led Pro- 

 fessor Drude to undertake the production of 

 the work before us. Hence the purpose of the 

 book is to supply a modern text embracing the 

 entire subject of optics, and to make possible 

 a deeper insight into the modem theory of 

 light. 



In order to attain this purpose, the author 

 omits most of the older historical references, 

 and gives only those later ones that will prove 

 useful to the reader in finding the more ex- 

 tended discussions in the periodical literature. 

 In fact, the greater part of the book treats of 

 the work done during the last fifteen or twenty 

 years. Hence it difFers essentially from other 

 treatises, many of which mention only the 

 work done previous to the last fifteen or twenty 

 years. Thus the discussion of the various 

 mechanical theories of light, with their per- 

 plexities as to a mechanically incompressible 

 ether and their shrewd and subtile attempts 

 to annihilate the longitudinal vibrations by 

 assumptions that lead to worse complications, 

 give place in this book to a presentation of 

 the electromagnetic theory, to a tracing of 

 that theory in its consequences, and to a dis- 

 cussion of the problems that are now being 

 solved or that ought to be solved in the near 

 future. 



In order to give the reader some idea of the 

 nature of the questions discussed, a few of 

 the more interesting ones will be mentioned. 

 In the chapter on physical conditions for 

 image formation the modem method of at- 

 tacking the problems of spherical and chro- 

 matic aberration is discussed. The chapter 



(in interference contains a presentation of the 

 manner in which interference is used for ob- 

 taining high spectroscopic resolution by in- 

 troducing a great difference of path between 

 the two interfering beams, and discusses the 

 problems of molecular vibrations to which this 

 use of interference leads. The chapter on 

 diffraction takes up not only the regular treat- 

 ment of the grating, etc., but also goes into 

 the question of resolving power in general, and 

 the limit of resolution of optical instruments. 

 In the chapter on absorbing media the reader 

 learns of the optical properties of the metals 

 and of the relations that have been established 

 and proposed between the optical and electrical 

 constants. The chapter on dispersion is par- 

 ticularly well done. Starting with the as- 

 sumption that the smallest particles of a body 

 possess natural periods of vibration, Professor 

 Drude shows how these natural periods of 

 the particles are involved with the period im- 

 pressed upon the body from without and the 

 index of refraction and the dielectric constant 

 in determining when the dispersion is normal 

 and when anomalous. He also shows how 

 from the observed dispersion, together with 

 other optical and electrical constants of a 

 substance, the position of the absorption bands 

 may be calculated. In the chapter on bodies 

 in motion the reader is introduced to the pres- 

 ent state of scientific opinion upon the ques- 

 tions: 'Is the ether at rest? Is its state of 

 rest disturbed by the motion of matter through 

 it ? ' His attention is also called to the points 

 that need further investigation and discussion. 

 The last portion of the book is given iip to 

 a presentation of the relations that have been 

 discovered between thermodynamics and 

 optics. Here important questions concerning 

 the efficiency of a source of light and the con- 

 version of other forms of energy into light are 

 discussed. The last chapter takes up the 

 properties of incandescent vapors and gases 

 and presents the electron theory, calculating 

 the probable size of an electron from optical 

 data. This last portion of the book con- 

 tains descriptions of work and theories that 

 are not discussed extensively in any other 

 English text. 



A glance at the list of questions just pre- 



