2i 4 SCIENCE PROGRESS. 



being content to have their specimens prepared for them. 

 There are, however, various advantages in making one's 

 own sections, and Behrens (12) has pointed out that useful 

 information about the constitution of a rock may be acquired 

 during the process of polishing. Different minerals, ac- 

 cording to their hardness and other physical properties, 

 begin to take the polish in a definite order : spinels, zircon, 

 tourmaline, quartz, olivine, magnetite, pyroxenes, epidote, 

 hornblende, etc.; and some of them show at that stage 

 characteristic appearances. 



Among new instruments adapted to penological work, 

 Dick's new form of polarising microscope (5), (13) has so 

 rapidly become popular that the mere mention of it will 

 suffice. With this instrument the observer can bring into 

 the centre of the field any small crystal in a rock-slice which 

 he wishes to investigate, and obtain its interference-figure 

 in convergent light without removing his eye from the 

 ocular. There is, further, a simple device to do away with 

 the difficulty of imperfect centring, the stage being here 

 kept fixed, while the polariser and analyser, connected by a 

 rack-movement, can be rotated together. Fedorow (14) 

 has devised a somewhat elaborate instrument for the optical 

 study of crystals, the chief feature of which is a provision for 

 rotating the object through measured angles about three rect- 

 angular axes. The applications of this apparatus belong 

 rather to physical optics and mineralogy, but it may also give 

 valuable aid in the study of rock-slices. For example, 

 in the methods of research to be noticed immediately, a 

 crystal-section lying not far out of a particular zone which 

 it is desired to study can be, so to speak, brought into that 

 zone by suitable rotation. 



Coming then to optical methods of procedure for the 

 identification of minerals, we must give the first place to 

 the discrimination of the various felspars, a problem which 

 may be regarded as the touch-stone of all attempts of this 

 kind. Various petrologists have made use for this purpose 

 of the "extinction-angles" given between crossed Nicols 

 with parallel light. In cases where crystal-grains of the 

 felspars can be readily isolated, the well-known method of 



