644 PETROLOGICAL ABSTRACTS AND REVIEWS 



parts of the book much more unified and gives a better appearance, but 

 will it be so easy for the student ? 



After a discussion of interference phenomena there is a new section on 

 the dispersion of birefringence and the departure of the interference 

 colors of crystals from the pure colors of Newton's scale. Section 46, on 

 the movement of light, has been entirely rewritten, and the figures have 

 been redrawn. The material in this portion of the work has been so 

 radically rearranged that it is difficult to compare it with the old edition, 

 especially since some of the material may have been transferred to the 

 future second part. 



In the preceding edition, following the theoretical discussion of the 

 intensity of transmitted light, came a chapter on the practical methods 

 of determining extinction angles. In the new edition this is omitted, 

 apparently to be given later, and the theoretical part is followed by an 

 explanation of the phenomena in convergent light, and there are given 

 the formulas for isochromatic curves, isogyres, and so on. The steps 

 in obtaining Neumann's formula for calculating the values of birefrin- 

 gence in any section have been increased from a half to four pages, a 

 very considerable help to the student who cannot refer to the original 

 article. The section on interference figures also has been much extended, 

 the explanation of the cross and rings obtained in uniaxial crystals alone 

 having been increased from four and one-half to nine and one-half pages. 

 Becke's skiodromes are given in illustration of both uniaxial and biaxial 

 figures. Under dispersion the old cuts have been discarded and new and 

 much better drawings, as well as photographs showing dispersion, have 

 been inserted. The subject of pleochroism has been extended from 

 twelve to sixteen pages, and a new section of two pages on luminescence 

 has been added. 



Polarizing prisms, which formerly came before the discussion of 

 microscopes, and before the discussion of interference, pleochroism, etc., 

 now comes near the end of the first half and takes up twenty-one pages. 

 Finally, there is the concluding section on Monochromatic Light, 

 increased from six to fourteen pages. Here are now given liquid color 

 filters, more material on monochromatic flames and the monochromator, 

 and a new description of the mercury lamp. 



The book is now, as it has always been in the past, the one big indis- 

 pensable work which begins where others end. Author and publisher 

 are to be congratulated on its appearance. Press work, paper, and illus- 

 trations are excellent and, in spite of the difiiculty of obtaining good 

 paper, are fully up to the standard of previous editions. 



