170 



and minerals, but is, or might be, of the greatest importance to 

 many workers in other fields, and especially to chemists and 

 analysts. The exj^lanation is very probably to be found in the 

 fact that the subject has become too extensive for satisfactory 

 treatment except in a special work. Such a work we now have 

 before us. It has been written by Dr. Weinschenk, Professor of 

 Petrography at Munich, and, whilst without any pretence to be 

 considered exhaustive, it seems to contain all that is necessary 

 for a good working knowledge of the subject. 



A general account of the polarising microscope, including notes 

 on various types of instruments by Seibert, Nachet, and Voigt & 

 Hochgesang, forms the first section of the book, and following 

 this a few pages are devoted to various adjustments of the stage, 

 crossed wires, and Nicol prisms, after which the author passes ta 

 the examination of crystals with ordinary light. But the 

 essential parts of the treatise are contained in the two following 

 sections, dealing with observations made with the help of parallel 

 and converging polarised light. We are here introduced to the 

 optical properties of crystals, the interference colours which they 

 exhibit, the methods of determining the direction, degree, and 

 relative speed of vibration of the two rays produced by doubly 

 refracting bodies, and also, of course, to the peculiarities of the 

 appearances of the "rings and brushes" of crystals. The 

 numerous pieces of subsidiary apparatus employed for various 

 purposes in connection with this part of the subject, such as the 

 various forms of stauroscope, the quarter-undulation plate, the 

 mica wedge, the quartz wedge, etc., are either discussed in the 

 text or referred to in special notes. The final section concerns 

 the peculiar twinning which occurs in many cases, and to some 

 optical anomalies. In a short appendix some of the wonderfully 

 improved forms of goniometer now in use are alluded to, and also^ 

 apparatus for the heating and delineation of crystals. 



The book, it must be remembered, is not a treatise on petrology 

 or crystallography, but simply what its title states it to be — an 

 introduction to the use of the polarising microscope — and as such 

 it appears to be an exceedingly useful and compact little work. 



D. J. S. 



Jvurn. Quekett Microscopical Club, Str. 2, Vol. VIJL, No. 49, Ifovember 1901. 



